Semiconductor device comprising gate electrode of one conductive layer and gate wiring of two conductive layers

ABSTRACT

A semiconductor device includes an oxide semiconductor layer provided over a substrate having an insulating surface; a gate insulating film covering the oxide semiconductor layer; a first conductive layer and a second conductive layer laminated in this order over the gate insulating film; an insulating film covering the oxide semiconductor layer and a gate wiring including a gate electrode (the first and second conductive layers); and a third conductive layer and a fourth conductive layer laminated in this order over the insulating film and electrically connected to the oxide semiconductor layer. The gate electrode is formed using the first conductive layer. The gate wiring is formed using the first conductive layer and the second conductive layer. A source electrode is formed using the third conductive layer. A source wiring is formed using the third conductive layer and the fourth conductive layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device provided with acircuit including a thin film transistor (hereinafter referred to as aTFT) using an oxide semiconductor film for a channel formation regionand a method for manufacturing the semiconductor device. For example,the present invention relates to an electronic appliance having as acomponent an electro-optical device typified by a liquid crystal displaypanel or a light-emitting display device including an organiclight-emitting element.

2. Description of the Related Art

As typically seen in a liquid crystal display device, a thin filmtransistor formed over a flat plate such as a glass substrate ismanufactured using amorphous silicon or polycrystalline silicon. A thinfilm transistor manufactured using amorphous silicon has low fieldeffect mobility, but can be formed over a large glass substrate. Incontrast, a thin film transistor manufactured using crystalline siliconhas high field effect mobility, but is not always suitable for beingformed over a large glass substrate due to a crystallization step suchas laser annealing.

In view of the foregoing, a technique by which a thin film transistor isformed using an oxide semiconductor and such a thin film transistor isapplied to an electronic device or an optical device has attractedattention. For example, Patent Document 1 and Patent Document 2 disclosea technique by which a thin film transistor is formed using zinc oxideor an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductorfilm and such a thin film transistor is used as a switching element orthe like of an image display device. Further, a technique by which anaperture ratio is increased with the use of light-transmittingelectrodes as gate electrodes and source and drain electrodes has beenconsidered (Patent Documents 3 and 4).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2007-123700-   [Patent Document 4] Japanese Published Patent Application No.    2007-81362

SUMMARY OF THE INVENTION

In general, a wiring for connecting elements such as transistors to eachother is formed by extending conductive layers for forming a gateelectrode and source and drain electrodes, whereby the wiring is formedin the same island as the conductive layers. Accordingly, a wiring forconnecting a gate of a transistor to a gate of another transistor (sucha wiring is called a gate wiring) is formed using the same layerstructure and material as a gate electrode of the transistor; and awiring for connecting a source of the transistor to a source of theanother transistor (such a wiring is called a source wiring) is formedusing the same layer structure and material as a source electrode of thetransistor, in many cases. Therefore, in the case where the gateelectrode and the source and drain electrodes are formed using alight-transmitting material, the gate wiring and the source wiring arealso formed using the light-transmitting material in many cases, likethe gate electrode and the source and drain electrodes.

However, in general, as compared to a material having a light-blockingproperty and a reflecting property, such as aluminum, molybdenum,titanium, tungsten, neodymium, copper, or silver, a light-transmittingconductive material such as indium tin oxide, indium zinc oxide, orindium tin zinc oxide has low conductivity. Accordingly, if a wiring isformed using a light-transmitting material, wiring resistance is high.For example, in the case where a large display device is manufactured,wiring resistance is significantly high because a wiring is long. Aswiring resistance increases, the waveform of a signal which istransmitted through the wiring becomes distorted, and a voltage drop dueto the wiring resistance results in a low voltage supply. Therefore, itis difficult to supply a normal voltage and a normal current, wherebynormal display and operation are difficult.

In addition, in terms of display performance, large capacitors andhigher aperture ratios are demanded for pixels. Pixels each having ahigh aperture ratio increase the use efficiency of light, so that powersaving and miniaturization of a display device can be achieved. Inrecent years, the size of pixels has been miniaturized and images withhigher definition are demanded. The miniaturization of the size ofpixels causes a decrease in the aperture ratio of the pixel because ofthe large formation area for transistors and wirings which occupies onepixel. Accordingly, in order to obtain a high aperture ratio in eachpixel in a regulation size, it is necessary to efficiently lay outcircuit components needed for the circuit configuration of the pixel.

In view of the foregoing problems, an object is to provide asemiconductor device with high aperture ratio and a manufacturing methodthereof. In addition, an object is to provide a semiconductor devicewith low power consumption and a manufacturing method thereof.

An embodiment of the invention to be disclosed is a semiconductor deviceincluding an oxide semiconductor layer provided over a substrate havingan insulating surface; a gate insulating film covering the oxidesemiconductor layer; a gate wiring including a gate electrode, beingformed by stacking a first conductive layer and a second conductivelayer in this order, and being provided over the gate insulating film;an insulating film covering the oxide semiconductor layer and the gatewiring including the gate electrode; and a source wiring including asource electrode, being formed by stacking a third conductive layer anda fourth conductive layer in this order, being provided over theinsulating film, and being electrically connected to the oxidesemiconductor layer. The gate electrode is formed using the firstconductive layer. The gate wiring is formed using the first conductivelayer and the second conductive layer. The source electrode is formedusing the third conductive layer. The source wiring is formed using thethird conductive layer and the fourth conductive layer.

Another embodiment of the invention to be disclosed is a semiconductordevice including an oxide semiconductor layer provided over a substratehaving an insulating surface; a gate insulating film covering the oxidesemiconductor layer; a gate wiring including a gate electrode, beingformed by stacking a first conductive layer and a second conductivelayer in this order, and being provided over the gate insulating film;an insulating film covering the oxide semiconductor layer and the gatewiring including the gate electrode; a source wiring including a sourceelectrode, being formed by stacking a third conductive layer and afourth conductive layer in this order, being provided over theinsulating film, and being electrically connected to the oxidesemiconductor layer; and a capacitor wiring. The gate electrode isformed using the first conductive layer. The gate wiring is formed usingthe first conductive layer and the second conductive layer. The sourceelectrode is formed using the third conductive layer. The source wiringis formed using the third conductive layer and the fourth conductivelayer. The capacitor wiring is formed using a fifth conductive layer anda sixth conductive layer.

Another embodiment of the invention to be disclosed is a semiconductordevice including an oxide semiconductor layer provided over a substratehaving an insulating surface; a gate insulating film covering the oxidesemiconductor layer; a gate wiring including a gate electrode, beingformed by stacking a first conductive layer and a second conductivelayer in this order, and being provided over the gate insulating film;an insulating film covering the oxide semiconductor layer and the gatewiring including the gate electrode; a source wiring including a sourceelectrode, being formed by stacking a third conductive layer and afourth conductive layer in this order, being provided over theinsulating film, and being electrically connected to the oxidesemiconductor layer; a capacitor wiring; and a storage capacitorportion. The gate electrode is formed using the first conductive layer.The gate wiring is formed using the first conductive layer and thesecond conductive layer. The source electrode is formed using the thirdconductive layer. The source wiring is formed using the third conductivelayer and the fourth conductive layer. The capacitor wiring is formedusing a fifth conductive layer and a sixth conductive layer. The storagecapacitor portion is formed using the oxide semiconductor layer, thethird conductive layer, the fifth conductive layer, the gate insulatingfilm, and the insulating film.

In the above, the first conductive layer and the third conductive layereach preferably have a light-transmitting property. Further, the secondconductive layer and the fourth conductive layer each preferably have alight-blocking property.

Further, in the above, the oxide semiconductor layer preferably containsat least one of indium, gallium, and zinc.

As an example of an oxide semiconductor that can be used in thisspecification, an oxide semiconductor denoted by InMO₃(ZnO)_(m) (m>0) isgiven. Here, M denotes one metal element or a plurality of metalelements selected from gallium (Ga), iron (Fe), nickel (Ni), manganese(Mn), and cobalt (Co). The case where Ga is selected as M includes thecase where Ga and any of the above metal elements other than Ga, such asNi or Fe, are selected as well as the case where only Ga is selected.Moreover, in the oxide semiconductor, in some cases, a transition metalelement such as Fe or Ni or an oxide of the transition metal iscontained as an impurity element in addition to a metal elementcontained as M. In this specification, of the above oxidesemiconductors, an oxide semiconductor containing at least gallium as Mis referred to as an In—Ga—Zn—O-based oxide semiconductor and a thinfilm using the material is referred to as an In—Ga—Zn—O-basednon-single-crystal film, in some cases.

Further, in the above, by using a multi-tone mask, a light-transmittingregion (a region with high light transmissivity) and a light-blockingregion (a region with low light transmissivity) can be formed with onemask (reticle). Accordingly, the light-transmitting region (the regionwith high light transmissivity) and the light-blocking region (theregion with low light transmissivity) can be formed without increasingthe number of masks.

Note that semiconductor devices in this specification mean all deviceswhich can function by utilizing semiconductor characteristics, andsemiconductor circuits, display devices, electro-optical devices,light-emitting display devices, and electronic appliances are allsemiconductor devices.

Note that a display device in this specification means an image displaydevice, a light-emitting device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP at the tip of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a chip on glass (COG) method.

According to an embodiment of the invention disclosed, alight-transmitting transistor or a light-transmitting capacitor can beformed. Therefore, even if a transistor or a capacitor is provided in apixel, the aperture ratio can be high because light can be transmittedalso in a portion where the transistor or the capacitor is formed.Further, since a wiring for connecting the transistor and an element(e.g., another transistor) or a wiring for connecting a capacitor and anelement (e.g., another capacitor) can be formed using a material withlow resistivity and high conductivity, the distortion of the waveform ofa signal and a voltage drop due to wiring resistance can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view of asemiconductor device;

FIGS. 2A to 2H are cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIGS. 3A to 3H are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 4A to 4F are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 5A to 5F are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIG. 6 is a cross-sectional view of a semiconductor device;

FIGS. 7A to 7C are a top view and cross-sectional views of asemiconductor device;

FIGS. 8A to 8C are a top view and cross-sectional views of asemiconductor device;

FIG. 9 is a top view of a semiconductor device;

FIGS. 10A and 10B are a top view and a cross-sectional view of asemiconductor device;

FIGS. 11A and 11B are a top view and a cross-sectional view of asemiconductor device;

FIG. 12 is a top view of a semiconductor device;

FIGS. 13A and 13B are a top view and a cross-sectional view of asemiconductor device;

FIGS. 14A to 14F are cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIGS. 15A to 15D are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 16A to 16D are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 17A to 17D are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 18A to 18D are cross-sectional views illustrating the method formanufacturing a semiconductor device;

FIGS. 19A1 to 19B2 are views of multi-tone masks;

FIGS. 20A and 20B are a top view and a cross-sectional view of asemiconductor device;

FIGS. 21A and 21B are diagrams of semiconductor devices;

FIGS. 22A and 22B are cross-sectional views of semiconductor devices;

FIG. 23 is a diagram of a pixel equivalent circuit of a semiconductordevice;

FIGS. 24A to 24C are cross-sectional views of semiconductor devices;

FIGS. 25A and 25B are a top view and a cross-sectional view of asemiconductor device;

FIGS. 26A1, 26A2, and 26B are top views and a cross-sectional view of asemiconductor device;

FIG. 27 is a cross-sectional view of a semiconductor device;

FIG. 28 is a cross-sectional view of a semiconductor device;

FIGS. 29A to 29D are views of electronic appliances;

FIGS. 30A and 30B are views of electronic appliances; and

FIGS. 31A and 31B are views of electronic appliances.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be described in detailwith reference to drawings. Note that the present invention is notlimited to the description below, and it is apparent to those skilled inthe art that modes and details can be modified in various ways withoutdeparting from the spirit of the present invention. The structuresaccording to different embodiments can be implemented in appropriatecombination. Note that in the structures of the present inventiondescribed below, like reference numerals refer to like portions orportions having similar functions, and the description thereof isomitted.

In this specification, a “film” means what has been formed over anentire surface and has not been patterned. A “layer” means what has beenpatterned to have a desired shape with the use of a resist mask or thelike. This distinction between “film” and “layer” is for convenience,and they are not particularly distinguished in some cases. Also as foreach layer in a layered film, the “film” and the “layer” are notparticularly distinguished in some cases.

Further, in this specification, a numeral such as “first”, “second”, or“third” which is included in a term is given for convenience in order todistinguish elements, does not limit the number, and does not limit thearrangement and the order of the steps.

Embodiment 1

In this embodiment, a semiconductor device and a manufacturing processthereof will be described with reference to FIGS. 1A and 1B, FIGS. 2A to2H, FIGS. 3A to 3H, FIGS. 4A to 4F, FIGS. 5A to 5F, FIG. 6, FIGS. 7A to7C, FIGS. 8A to 8C, FIG. 9, FIGS. 10A and 10B, FIGS. 11A and 11B, andFIG. 12.

FIGS. 1A and 1B illustrate a semiconductor device according to thisembodiment. FIG. 1A is a top view and FIG. 1B is a cross-sectional viewtaken along line A-B of FIG. 1A.

A semiconductor device illustrated in FIG. 1A includes a pixel portionwhich has a gate wiring and a capacitor wiring provided in a direction1, a source wiring provided in a direction 2, which intersects with thegate wiring and the capacitor wiring, and a transistor 150 a in thevicinity of an intersection of the gate wiring and the source wiring.Note that in this specification, the pixel portion refers to a regionsurrounded by a plurality of gate wirings and a plurality of sourcewirings.

The transistor 150 a illustrated in FIGS. 1A and 1B is a so-calledtop-gate transistor including, over a substrate 100 having an insulatingsurface, an oxide semiconductor layer 103 a, a gate insulating film 104covering the oxide semiconductor layer 103 a, a conductive layer 109 afunctioning as a gate electrode and being provided over the gateinsulating film 104, an insulating film 112 covering the oxidesemiconductor layer 103 a and the conductive layer 109 a, and conductivelayers 117 a and 117 b and functioning as source and drain electrodesand being provided over the insulating film 112 and electricallyconnected to the oxide semiconductor layer 103 a.

Further, as for the transistor 150 a, the oxide semiconductor layer 103a, the conductive layer 109 a functioning as a gate electrode, and theconductive layers 117 a and 117 b functioning as source and drainelectrodes are formed using light-transmitting materials. By thusforming the oxide semiconductor layer 103 a, the gate electrode, and thesource and drain electrodes of the transistor 150 a with the use oflight-transmitting materials, light can be transmitted also in a portionwhere the transistor is formed; therefore, the aperture ratio of a pixelcan be improved.

The gate wiring electrically connected to the gate electrode of thetransistor 150 a is formed by stacking the conductive layer 109 a havinga light-transmitting property and the conductive layer 111 a having alight-blocking property in this order, and the source wiringelectrically connected to the source or drain electrode of thetransistor 150 a is formed by stacking the conductive layer 117 a havinga light-transmitting property and a conductive layer 119 a having alight-blocking property in this order. That is to say, the gateelectrode of the transistor 150 a is formed using part of the conductivelayer 109 a having a light-transmitting property, which is included inthe gate wiring, and the source or drain electrode is formed using partof the conductive layer 117 a having a light-transmitting property,which is included in the source wiring.

By stacking the light-transmitting conductive layer and thelight-blocking conductive layer in this order to form each of the gatewiring and the source wiring, wiring resistance and power consumptioncan be reduced. In addition, since the gate wiring and the source wiringare each formed using the light-blocking conductive layer, a spacebetween pixels can be shielded from light. That is, with the gatewirings provided in a row direction and the source wirings provided in acolumn direction, the space between the pixels can be shielded fromlight without using a black matrix.

Further, the capacitor wiring is provided in the direction 1 which isthe same as that of the gate wiring. A portion of the capacitor wiring,which is in a pixel region, is desirably formed using a conductive layer109 b having a light-transmitting property and a portion of thecapacitor wiring, which is overlapped with the source wiring, may beformed by stacking the conductive layer 109 b having alight-transmitting property and a conductive layer 111 b having alight-blocking property in this order. A storage capacitor portion 151 ais formed in the capacitor wiring. The storage capacitor portion 151 ais connected to the source or drain electrode of the transistor 150 a.The storage capacitor portion 151 a includes the gate insulating film104 and the insulating film 112 functioning as dielectrics and the oxidesemiconductor layer 103 b, the conductive layer 109 b, and theconductive layer 117 b functioning as electrodes.

In this embodiment, an example is described in which the width of thecapacitor wiring and the gate wiring are equal to each other; however,the width of the capacitor wiring and the width of the gate wiring maybe different. The width of the capacitor wiring is preferably largerthan that of the gate wiring. When the width of the capacitor wiring islarge, the area of the storage capacitor portion 151 a can be large.

By thus forming the storage capacitor portion 151 a using the oxidesemiconductor layer 103 b, the conductive layer 109 b having alight-transmitting property, and the conductive layer 117 b, light canbe transmitted also in a portion where the storage capacitor portion 151a is formed. Therefore, the aperture ratio can be improved. Further, bybeing formed using the light-transmitting conductive layer, the storagecapacitor portion 151 a can be formed to be large without reducing theaperture ratio. Therefore, even when the transistor is off, potentialholding characteristics of a pixel electrode can be favorable and thusdisplay quality can be favorable. Further, a feedthrough potential canbe low.

Further, the transistor 150 a illustrated in FIGS. 1A and 1B can be usedas a pixel transistor provided in a pixel portion of a light-emittingdisplay device typified by a liquid crystal display device or an ELdisplay device. Therefore, in the illustrated example, a contact hole126 is formed in the insulating film 120, a pixel electrode layer (aconductive layer 122 b having a light-transmitting property) is formedover the insulating film 120, and the pixel electrode layer (theconductive layer 122 b having a light-transmitting property) and theconductive layer 117 b are connected to each other through the contacthole 126 formed in the insulating film 120.

Next, an example of a manufacturing process of a semiconductor devicewill be described with reference to FIGS. 2A to 2H, FIGS. 3A to 3H,FIGS. 4A to 4F, and FIGS. 5A to 5F.

First, an oxide semiconductor film 101 is formed over the substrate 100having an insulating surface (see FIGS. 2A and 2B).

As the substrate 100 having an insulating surface, a visiblelight-transmitting glass substrate used for a liquid crystal displaydevice or the like can be used, for example. The glass substrate ispreferably a non-alkali glass substrate. As a material of the non-alkaliglass substrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used.Alternatively, an insulating substrate which is formed of an insulator,such as a ceramic substrate, a quartz substrate, or a sapphiresubstrate; a semiconductor substrate which is formed of a semiconductormaterial such as silicon and whose surface is covered with an insulatingmaterial; a conductive substrate which is formed of a conductor such asmetal or stainless steel and whose surface is covered with an insulatingmaterial; or the like may be used as the substrate 100 having aninsulating surface.

An insulating film serving as a base film may be formed over thesubstrate 100 having an insulating surface. The insulating film has afunction of preventing diffusion of impurities such as alkali metal (Li,Cs, Na, or the like), alkaline earth metal (Ca, Mg, or the like), or anyother metal element from the substrate 100. Note that the concentrationof Na is 5×10¹⁹/cm³ or lower, preferably 1×10¹⁸/cm³ or lower. Theinsulating film can be formed to have a single-layer structure of asilicon nitride film, a silicon oxide film, a silicon nitride oxidefilm, a silicon oxynitride film, an aluminum oxide film, an aluminumnitride film, an aluminum oxynitride film, or an aluminum nitride oxidefilm or a layered structure of any of the above films.

The oxide semiconductor film 101 can be formed using an In—Ga—Zn—O-basednon-single-crystal film. For example, the oxide semiconductor film 101is formed by a sputtering method using a target of an oxidesemiconductor containing In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). Theconditions for sputtering can be, for example, as follows: the distancebetween the substrate 100 and the target is 30 mm to 500 mm; thepressure is 0.1 Pa to 2.0 Pa; the DC power is 0.25 kW to 5.0 kW (in thecase where the target is 8 inch in diameter); and the atmosphere is anargon atmosphere, an oxygen atmosphere, or a mixed atmosphere of argonand oxygen. Note that as the oxide semiconductor film, a ZnO-basednon-single-crystal film may be used. Further, the thickness of the oxidesemiconductor film 101 may be about 5 nm to 200 nm.

As a sputtering method, employed can be an RF sputtering method in whicha high-frequency power supply is used as a sputtering power supply, a DCsputtering method, or a pulsed DC sputtering method in which a DC biasis applied in a pulsed manner. An RF sputtering method is mainlyemployed in the case of forming an insulating film, and a DC sputteringmethod is mainly used in the case of forming a metal film.

Note that in the case where the insulating film is formed, plasmatreatment may be performed on a surface of the insulating film beforethe oxide semiconductor film 101 is formed. By performing plasmatreatment, dust attached to a surface of the insulating film can beremoved.

A pulsed DC power supply is preferably used because dust can be reducedand the film thickness distribution can be uniform. Further, the oxidesemiconductor film 101 is formed without being exposed to the air afterthe plasma treatment is performed, so that attachment of dust ormoisture to the interface between the insulating film and the oxidesemiconductor film 101 can be suppressed.

A multi-source sputtering apparatus in which a plurality of targets ofdifferent materials can be placed may be used. With the multi-sourcesputtering apparatus, different films can be formed to be stacked in thesame chamber, or a film can be formed by sputtering a plurality of kindsof materials at the same time in the same chamber. Alternatively, amethod using a magnetron sputtering apparatus provided with a magneticfield generating mechanism inside the chamber (magnetron sputteringmethod), an ECR sputtering method using plasma generated with the use ofmicrowaves, or the like may be employed. Still alternatively, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a compound thereof, a bias sputtering method in which a voltage isapplied also to a substrate during deposition, or the like may beemployed.

Next, resist masks 102 a and 102 b are formed over the oxidesemiconductor film 101 and the oxide semiconductor film 101 isselectively etched using the resist masks 102 a and 102 b, so thatisland-shaped oxide semiconductor layers 103 a and 103 b are formed (seeFIGS. 2C and 2D). In the case of forming the resist masks by a spincoating method, large quantities of resist materials and a large amountof developing solution are used in order to improve uniformity of aresist film; thus, large quantities of surplus materials are consumed.In a film formation method using a spin coating method, the increase insize of a substrate will be particularly disadvantageous in massproduction because a mechanism for rotating a large substrate is largeand a loss and waste amount of a material liquid are large. Moreover,when a film is formed by spin-coating a rectangular substrate, circularunevenness is likely to appear on the film with a rotating axis as acenter. Therefore, it is preferable to form the resist masks byselectively forming a resist material film by a droplet discharge methodsuch as an ink-jet method, a screen printing method, or the like andexposing the resist material film to light. By selectively forming aresist material film, the usage of resist materials can be reduced andthus significant cost reduction can be achieved. Accordingly, a largesubstrate having a size of 1000×1200 mm, 1100×1250 mm, 1150×1300 mm, orthe like can be used.

Either wet etching or dry etching can be employed as an etching methodin this case. Here, an unnecessary portion of the oxide semiconductorfilm 101 is removed by wet etching using a mixed solution of aceticacid, nitric acid, and phosphoric acid, so that island-shaped oxidesemiconductor layers 103 a and 103 b are formed. Note that the resistmasks 102 a and 102 b are removed after the etching. Further, an etchantused for wet etching is not limited to the above as long as it can etchthe oxide semiconductor film 101. In the case of performing dry etching,a gas containing chlorine or a gas containing chlorine to which oxygenis added is preferably used. By using a gas containing chlorine andoxygen, the etching selectivity of the insulating film serving as a basefilm to the oxide semiconductor film 101 is likely to be high and thus,the insulating film can be sufficiently prevented from being damaged.

Further, an etching apparatus for which a reactive ion etching method(RIE method) is employed or a dry etching apparatus for which ahigh-density plasma source such as ECR (electron cyclotron resonance) orICP (inductivity coupled plasma) is used can be used for dry etching.Furthermore, as a dry etching apparatus by which electric discharge islikely to be homogeneous in a large area as compared to the case of anICP etching apparatus, there is an ECCP (enhanced capacitively coupledplasma) mode etching apparatus in which an upper electrode is grounded,a high-frequency power source of 13.56 MHz is connected to a lowerelectrode, and a low-frequency power source of 3.2 MHz is connected tothe lower electrode. This ECCP mode etching apparatus can be applied,for example, even when a substrate of the tenth generation with a sideof larger than 3 m is used.

After that, heat treatment at 200° C. to 600° C., typically 300° C. to500° C., is preferably performed. Here, heat treatment is performed in anitrogen atmosphere at 350° C. for an hour. This heat treatment involvesthe rearrangement of the In—Ga—Zn—O-based oxide semiconductor used forforming the oxide semiconductor layers 103 a and 103 b at the atomiclevel. This heat treatment (including light annealing) is importantbecause the strain that inhibits the movement of carriers in the oxidesemiconductor layers 103 a and 103 b can be released by the heattreatment. Note that the timing when the heat treatment is performed isnot particularly limited as long as it is after the formation of theoxide semiconductor layers 103 a and 103 b.

Next, a gate insulating film 104 is formed over the island-shaped oxidesemiconductor layers 103 a and 103 b and then, a conductive film 105 isformed over the gate insulating film 104 (see FIGS. 2E and 2F).

The gate insulating film 104 can be formed to have a single-layerstructure of a silicon oxide film, a silicon oxynitride film, a siliconnitride film, a silicon nitride oxide film, an aluminum oxide film, analuminum nitride film, an aluminum oxynitride film, an aluminum nitrideoxide film, or a tantalum oxide film or a layered structure of any ofthe above films. The gate insulating film 104 can be formed to athickness from 50 nm to 250 nm by a sputtering method or the like. Forexample, a 100-nm-thick oxide silicon film may be formed as the gateinsulating film 104 by a sputtering method. Alternatively, a100-nm-thick aluminum oxide film may be formed by a sputtering method.Note that the gate insulating film 104 preferably has alight-transmitting property.

By forming the gate insulating film 104 using a dense film, moisture oroxygen can be prevented from entering the oxide semiconductor layers 103a and 103 b from the substrate 100 side. Further, impurities such asalkali metal (Li, Cs, Na, or the like), alkaline earth metal (Ca, Mg, orthe like), or any other metal elements, which are contained in thesubstrate 100, can be prevented from entering the oxide semiconductorlayers from the substrate 100 side. Note that the concentration of Na is5×10¹⁹/cm³ or lower, preferably 1×10¹⁸/cm³ or lower. Thus, a change insemiconductor characteristics of a semiconductor device using the oxidesemiconductor can be suppressed. Further, reliability of thesemiconductor device can be increased.

As the conductive film 105, indium tin oxide (ITO), indium tin oxidecontaining silicon oxide (ITSO), organic indium, organic tin, zinc oxide(ZnO), titanium nitride, or the like can be used. Alternatively, indiumzinc oxide (IZO) containing zinc oxide, zinc oxide doped with gallium(Ga), tin oxide (SnO₂), indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, or the like may beused. Such a material can be used to form the conductive film 105 with asingle-layer structure or a layered structure by a sputtering method.However, in the case of the layered structure, the light transmissivityof each of a plurality of films is desirably sufficiently high.

Next, resist masks 107 a and 107 b are formed over the conductive film105 and the conductive film 105 is selectively etched using the resistmasks 107 a and 107 b, so that conductive layers 109 a and 109 b areformed (see FIGS. 2G and 2H). Note that the resist masks 107 a and 107 bare removed after the etching. In this case, to increase coverage of theinsulating film 112 to be formed later and prevent breakage of theinsulating film 112, the etching is preferably performed so that endportions of the gate electrode have tapered shapes. Note that the gateelectrode includes the electrode and the wiring formed using theconductive film, such as the gate wiring.

Next, a conductive film 106 is formed over the gate insulating film 104and the conductive layers 109 a and 109 b (see FIGS. 3A and 3B).

The conductive film 106 can be formed to have a single-layer structureor a layered structure using a metal material such as aluminum (Al),tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel(Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese(Mn), or neodymium (Nd), an alloy material containing any of the abovemetal materials as its main component, or a nitride containing any ofthe above metal materials as its component. It is desirable to use a lowresistance conductive material such as aluminum.

When the conductive film 106 is formed over the conductive film 105 (orthe conductive layers 109 a and 109 b), both the films react with eachother in some cases. For example, when the conductive film 105 is formedusing ITO and the conductive film 106 is formed using aluminum, achemical reaction occurs therebetween. Accordingly, to avoid such achemical reaction, a refractory material is preferably sandwichedbetween the conductive film 105 and the conductive film 106. Forexample, as the refractory material, molybdenum, titanium, tungsten,tantalum, chromium, and the like can be given. Further, it is preferableto form the conductive film 106 to be a multi-layer film by using amaterial with high conductivity over a film formed using the refractorymaterial. As the material with high conductivity, aluminum, copper,silver, and the like can be given. For example, in the case where theconductive film 106 is formed to have a layered structure, a stack ofmolybdenum as a first layer, aluminum as a second layer, and molybdenumas a third layer, or a stack of molybdenum as a first layer, aluminumcontaining a small amount of neodymium as a second layer, and molybdenumas a third layer can be used.

Next, a resist mask 110 is formed over the conductive film 106 and theconductive film 106 is etched using the resist mask 110 (see FIGS. 3Cand 3D). The resist mask 110 is removed after the etching. Accordingly,part of the conductive film 106, over which the resist mask 110 is notformed, is removed, so that the conductive layer 109 a is exposed. Thus,the surface areas of the conductive layer 111 a and the conductive layer109 a are different from each other. That is, the surface area of theconductive layer 109 a is larger than that of the conductive layer 111a. Alternatively, as for the conductive layers 111 a and 109 a, thereare a region in which the conductive layers 111 a and 109 a areoverlapped with each other and a region in which the conductive layers111 a and 109 a are not overlapped with each other.

A region including at least the conductive layer 111 a having alight-blocking property functions as the gate wiring and a regionincluding the conductive layer 109 a having a light-transmittingproperty functions as the gate electrode. By forming the conductivelayer 109 a functioning as the gate electrode with the use of alight-transmitting material, light can be transmitted also in a portionwhere the gate electrode is formed; therefore, the aperture ratio of apixel can be improved. Further, by forming the conductive layer 111 afunctioning as a gate wiring with the use of a light-blocking conductivelayer, wiring resistance and power consumption can be reduced. Further,since the gate wiring is formed using the light-blocking conductivelayer, a space between pixels can be shielded from light. Further, acontrast can be improved.

Note that although the steps in which the conductive layers 109 a and109 b are formed and then the conductive layer 111 a having alight-blocking property is formed are described, the order of formationmay be inverted. That is, after the conductive layer 111 a having alight-blocking property which functions as the gate wiring is formed,the conductive layers 109 a and 109 b each having a light-transmittingproperty which function as the gate electrodes may be formed (see FIGS.8A and 8C).

Further, as illustrated in FIGS. 3C and 3D, the capacitor wiring isprovided in the same direction as that of the gate wiring. Although partof the capacitor wiring, which is in a pixel region, is desirably formedusing the conductive layer 109 b having a light-transmitting property,part of the capacitor wiring, which is overlapped with the source wiringto be formed later may be formed by stacking the conductive layer 109 bhaving a light-transmitting property and the conductive layer 111 bhaving a light-blocking property in this order (see FIG. 1A). With sucha structure, resistance can be reduced.

Although in this embodiment, an example is described in which thecapacitor wiring and the gate wiring are formed so as to have an equalwidth, the capacitor wiring and the gate wiring may have differentwidths. The width of the capacitor wiring is preferably larger than thatof the gate wiring. The surface area of the storage capacitor portion151 a can be increased.

Note that treatment for increasing conductivity in part of or wholeregions of the oxide semiconductor layers 103 a and 103 b may beperformed after formation of the oxide semiconductor layers 103 a and103 b, after formation of the gate insulating film 104, or afterformation of the gate electrode. For example, hydrogenation treatmentcan be given as the treatment for increasing conductivity. By providingsilicon nitride containing hydrogen in an upper layer of the oxidesemiconductor layer 103 b and applying heat, the oxide semiconductorlayer can be hydrogenated. Alternatively, by applying heat in a hydrogenatmosphere, hydrogenation may be performed. Alternatively, asillustrated in FIG. 6, by forming a channel protective layer 127 in aregion overlapping a channel formation region of the oxide semiconductorlayer 103 a, a region where conductivity is increased can be selectivelyformed in the oxide semiconductor layer 103 a.

The channel protective layer 127 is desirably formed using siliconoxide. By forming the channel protective layer 127 using silicon oxide,entry of hydrogen into a channel portion of the oxide semiconductorlayer 103 a can be suppressed. Note that the channel protective layer127 may be removed after the treatment for increasing conductivity isperformed. Alternatively, the channel protective layer 127 may be formedusing a resist. In this case, the resist is preferably removed afterhydrogenation treatment. By thus performing the treatment for increasingconductivity on the oxide semiconductor layers 103 a and 103 b, acurrent of a transistor can flow easily and thus resistance of anelectrode of a capacitor can be reduced.

Although FIG. 6 illustrates an example in which the channel protectivelayer 127 is formed in contact with the oxide semiconductor layer 103 a,the channel protective layer 127 may be provided over the gateinsulating film 104. Further, by adjusting the shapes of the channelprotective layer 127 and the conductive layer 109 a functioning as thegate electrode so that the channel protective layer 127 is larger thanthe conductive layer 109 a, an offset region can be formed.

Next, after the insulating film 112 functioning as an interlayerinsulating film is formed so as to cover the conductive layers 109 a and109 b and the gate insulating film 104, contact holes reaching the oxidesemiconductor layer are formed in the insulating film 112 so that partsof a surface of the oxide semiconductor layer are exposed (see FIGS. 3Eand 3F).

The insulating film 112 can be formed to have a single-layer or layeredstructure using any of an insulating film containing oxygen or nitrogen,such as silicon oxide, silicon nitride, silicon oxynitride, or siliconnitride oxide; a film containing carbon such as DLC (diamond-likecarbon); and a film formed using an organic material such as epoxy,polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic ora siloxane material such as a siloxane resin. Note that the insulatingfilm 112 preferably has a light-transmitting property.

Next, a conductive film 113 is formed over the insulating film 112 (seeFIGS. 3G and 3H).

The conductive film 113 is desirably formed using a materialsubstantially the same as that used for the conductive film 105. Thematerial substantially the same as that of the conductive film 105 meansa material whose element of a main component is the same as that of thematerial used for the conductive film 105. In terms of impurities, thekinds, the concentrations, and the like of elements contained aredifferent in some cases. In this manner, when the conductive film 113 isformed using the material substantially the same as that of theconductive film 105 by sputtering or evaporation, there is an advantagethat the material can be shared between the conductive films 105 and113. When the material can be shared, the same manufacturing apparatuscan be used, manufacturing steps can proceed smoothly, and throughputcan be improved, which lead to cost reduction.

Next, resist masks 115 a and 115 b are formed over the conductive film113 and the conductive film 113 is selectively etched using the resistmasks 115 a and 115 b, so that conductive layers 117 a and 117 b areformed (see FIGS. 4A and 4B). Note that after the etching, the resistmasks 115 a and 115 b are removed.

Next, a conductive film 114 is formed over the conductive layers 117 aand 117 b and the insulating film 112 (see FIGS. 4C and 4D).

The conductive film 114 can be formed to have a single-layer structureor a layered structure using a metal material such as aluminum (Al),tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel(Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese(Mn), or neodymium (Nd), an alloy material containing any of the abovemetal materials as its main component, or a nitride containing any ofthe above metal materials as its component. It is desirable to use a lowresistance conductive material such as aluminum.

Further, the conductive film 114 is desirably formed using a materialdifferent from that used for the conductive film 106. Alternatively, theconductive film 114 is desirably formed to have a layered structuredifferent from that of the conductive film 106. This is because inmanufacturing steps of a semiconductor device, temperatures of heatapplied to the conductive film 114 and the conductive film 106 aredifferent from each other in many cases. In general, the conductive film106 tends to have a higher temperature. Accordingly, the conductive film106 is desirably formed using a material or layered structure with ahigher melting point. Alternatively, the conductive film 106 isdesirably formed using a material or layered structure in which hillocksare less likely to occur. Alternatively, since the conductive film 114is included in a signal line through which a video signal is supplied insome cases, the conductive film 114 is desirably formed using a materialor layered structure having wiring resistance lower than that of theconductive film 106.

When the conductive film 114 is formed over the conductive film 113 (orthe conductive layers 117 a and 117 b) as in the case where theconductive film 106 is formed over the conductive film 105 (or theconductive layers 109 a and 109 b), both the films react with each otherin some cases. Thus, also in the case where the conductive film 114 isformed over the conductive film 113, a refractory material is desirablysandwiched between the conductive film 113 and the conductive film 114.For example, as the refractory material, molybdenum, titanium, tungsten,tantalum, chromium, and the like can be given. Further, it is preferableto form the conductive film 114 to be a multi-layer film by using amaterial with high conductivity over a film formed using the refractorymaterial. As the material with high conductivity, aluminum, copper,silver, and the like can be given.

Next, a resist mask 118 is formed over the conductive film 114 and theconductive film 114 is etched using the resist mask 118 (see FIGS. 4Eand 4F). The resist mask 118 is removed after the etching. Accordingly,part of the conductive film 114, over which the resist mask 118 is notformed, is removed, so that the conductive layer 117 a is exposed. Thus,the surface areas of the conductive layer 119 a and the conductive layer117 a are different from each other. That is, the surface area of theconductive layer 117 a is larger than that of the conductive layer 119a. Alternatively, as for the conductive layers 119 a and 117 a, thereare a region in which the conductive layers 119 a and 117 a areoverlapped with each other and a region in which the conductive layers119 a and 117 a are not overlapped with each other.

A region including at least the conductive layer 119 a having alight-blocking property functions as the source wiring and a regionincluding the conductive layer 117 a having a light-transmittingproperty functions as the source or drain electrode. By forming theconductive layers 117 a and 117 b functioning as source and drainelectrodes with the use of a light-transmitting conductive layer, lightcan be transmitted also in a portion where the source or drain electrodeis formed; therefore, the aperture ratio of a pixel can be improved.Further, by forming the conductive layer 119 a functioning as the sourcewiring with the use of the light-blocking conductive layer, wiringresistance and power consumption can be reduced. Further, since thesource wiring is formed using the conductive layer 119 a having alight-blocking property, a space between pixels can be shielded fromlight. That is, with the gate wirings provided in a row direction andthe source wirings provided in a column direction, a space betweenpixels can be shielded from light without using a black matrix.

Note that although the steps in which the conductive layers 117 a and117 b are formed and then the conductive layer 119 a having alight-blocking property is formed are described, the order of formationmay be inverted. That is, after the conductive layer 119 a having alight-blocking property which functions as the source wiring is formed,the conductive layers 117 a and 117 b each having a light-transmittingproperty which function as the source and drain electrodes may be formed(see FIGS. 8A and 8B).

Further, in FIGS. 4E and 4F, the conductive layer 117 b functions alsoas an electrode of the storage capacitor portion 151 a. In the capacitorwiring, the storage capacitor portion 151 a includes the gate insulatingfilm 104 and the insulating film 112 functioning as dielectrics and theoxide semiconductor layer 103 b, the conductive layer 109 b, and theconductive layer 117 b functioning as electrodes. With such a structure,resistance can be reduced.

By thus forming the storage capacitor portion 151 a using thelight-transmitting conductive layer, light can be transmitted also in aportion where the storage capacitor portion 151 a is formed. Therefore,the aperture ratio can be improved. Further, when the storage capacitorportion 151 a is formed using the light-transmitting material, thestorage capacitor portion 151 a can be large. Thus, even when thetransistor is off, potential holding characteristics of a pixelelectrode can be favorable and thus display quality can be favorable.Further, a feedthrough potential can be low.

In this manner, the transistor 150 a and the storage capacitor portion151 a can be formed. Further, the transistor 150 a and the storagecapacitor portion 151 a can be light-transmitting elements. Note that inthe case where the storage capacitor portion is formed using the oxidesemiconductor layer 103 b and the gate insulating film 104 as adielectric, a potential of the capacitor wiring formed using theconductive layer 109 b can be higher than a potential of a counterelectrode (a potential of a common line). Electric charges of the oxidesemiconductor layer 103 b can be induced and thus the oxidesemiconductor layer 103 b can function as an electrode of the storagecapacitor portion. On the other hand, in the case where the storagecapacitor portion is formed without using the oxide semiconductor layer103 b or the case where the oxide semiconductor layer 103 b is subjectedto treatment for increasing conductivity such as hydrogenationtreatment, the potential of the capacitor wiring may be equal to that ofthe counter electrode (common electrode). Thus, the number of wiringscan be reduced.

Next, after the insulating film 120 is formed, a resist mask (notillustrated) is formed over the insulating film 120, and the insulatingfilm 120 is etched using the resist mask to form a contact hole in theinsulating film 120 (see FIGS. 5A and 5B). The insulating film 120 canserve as an insulating film planarizing a surface over which thetransistor 150 a, the storage capacitor portion 151 a, the wiring, orthe like is formed. Since the transistor 150 a and the storage capacitorportion 151 a can be formed as light-transmitting elements, regionswhere they are provided can also be utilized as opening regions.Therefore, it is advantageous to relieve unevenness due to thetransistor 150 a, the storage capacitor portion 151 a, the wiring, orthe like, so that an upper portion over which these elements are formedis planarized.

Further, the insulating film 120 can serve as an insulating film whichprotects the transistor 150 a from impurities or the like. Theinsulating film 120 can be formed using, for example, a film containingsilicon nitride. A film containing silicon nitride is preferable becauseit is highly effective in blocking impurities. Alternatively, theinsulating film 120 may be formed using a film containing an organicmaterial. As examples of the organic material, acrylic, polyimide,polyamide, and the like are preferable. Such organic materials arepreferable in terms of a high functionality of flattening unevenness.Accordingly, in the case where the insulating film 120 is formed to havea layered structure of a film containing silicon nitride and a filmcontaining an organic material, it is preferable to provide the filmcontaining silicon nitride and the film containing an organic materialin the lower side and in the upper side, respectively. Note that in thecase where the insulating film 120 is formed to have a layeredstructure, the light transmittance of each of films is preferablysufficiently high. Alternatively, a photosensitive material may be used.In this case, it is not necessary to etch the insulating film 120 toform a contact hole.

Further, the insulating film 120 can serve as a color filter. When acolor filter is provided on the substrate 100 side, it is not necessaryto provide a color filter on the counter substrate side. Therefore, amargin for adjusting the positions of two substrates is not necessary,which can facilitate manufacture of a panel. Note that the insulatingfilm 120 is not necessarily formed. The pixel electrode may be formedover the same layer as the source electrode and the source wiring.

Next, a conductive film 121 is formed over the insulating film 120 andthe contact hole (see FIGS. 5C and 5D). The conductive film 121 isdesirably formed using a material substantially the same as that usedfor forming the conductive film 105 and the conductive film 113. In thismanner, when the conductive film 121 is formed using the materialsubstantially the same as that of the conductive film 105 and theconductive film 113 by sputtering or evaporation, there is an advantagethat the material can be shared among the conductive films 105 and 113and the conductive film 121. When the material can be shared, the samemanufacturing apparatus can be used, manufacturing steps can proceedsmoothly, and throughput can be improved, which lead to cost reduction.

Next, a resist mask (not illustrated) is formed over the conductive film121, and the conductive film 121 is selectively etched using the resistmask, so that conductive layers 122 a and 122 b are formed (see FIGS. 5Eand 5F). Note that the resist mask is removed after the etching.

The conductive layers 122 a and 122 b can function as pixel electrodes.Further, the conductive layers 122 a and 122 b can connect the sourcewiring, the source electrode, the gate wiring, the gate electrode, thepixel electrode, the capacitor wiring, the electrode of the storagecapacitor portion, and the like to each other through the contact hole.Therefore, the conductive layers 122 a and 122 b can function as wiringsfor connecting conductors.

As described above, a semiconductor device can be manufactured.According to the manufacturing method described in this embodiment, thetransistor 150 a having a light-transmitting property and the storagecapacitor portion 151 a having a light-transmitting property can beformed. Therefore, even if a transistor or a storage capacitor portionis provided in a pixel, the aperture ratio can be high because light canbe transmitted also in a portion where the transistor or the storagecapacitor portion is formed. Further, since a wiring for connecting thetransistor and an element (e.g., another transistor) can be formed usinga material with low resistivity and high conductivity, the distortion ofthe waveform of a signal and a voltage drop due to wiring resistance canbe reduced.

Next, another example of a semiconductor device will be described withreference to FIGS. 7A to 7C. Note that many portions are common to thesemiconductor device illustrated in FIGS. 7A to 7C and the semiconductordevice in FIGS. 1A and 1B. Therefore, description of common portions isomitted and different points will be described. Further, FIG. 7A is aplan view, FIG. 7B is a cross-sectional view taken along A-B in FIG. 7A,and FIG. 7C is a cross-sectional view taken along C-D in FIG. 7A.

In FIGS. 1A and 1B, an example is illustrated in which the gate wiringand the source wiring are each formed by stacking a light-blockingconductive layer over a light-transmitting conductive layer; however,the gate wiring and the source wiring may be formed by stacking alight-blocking conductive layer and a light-transmitting conductivelayer in this order (see FIGS. 7A to 7C). The conductive layer 109 ahaving a light-transmitting property, which functions as a gateelectrode, may be connected to the conductive layer 111 a having alight-blocking property, which functions as a gate wiring. Like the gatewiring, the conductive layer 117 a having a light-transmitting property,which functions as a source or drain electrode, may be connected to theconductive layer 119 a having a light-blocking property, which functionsas a source wiring.

Next, another example of a semiconductor device will be described withreference to FIGS. 8A to 8C. Note that many portions are common to thesemiconductor device illustrated in FIGS. 8A to 8C and the semiconductordevice in FIGS. 1A and 1B. Therefore, description of common portions isomitted and different points will be described. Further, FIG. 8A is aplan view, FIG. 8B is a cross-sectional view taken along A-B in FIG. 8A,and FIG. 8C is a cross-sectional view taken along C-D in FIG. 8A.

In FIGS. 1A and 1B, an example is illustrated in which the gate wiringand the source wiring are each formed by stacking a light-transmittingconductive layer and a light-blocking conductive layer in this order;however, the gate wiring and the source wiring may be formed using alight-blocking conductive layer (see FIGS. 8A to 8C). The conductivelayer 109 a having a light-transmitting property, which functions as agate electrode, may be connected to the conductive layer 111 a having alight-blocking property, which functions as a gate wiring. Like the gatewiring, the conductive layer 117 a having a light-transmitting property,which functions as a source or drain electrode, may be connected to theconductive layer 119 a having a light-blocking property, which functionsas a source wiring.

Further, in the case where the transistor is formed over the gatewiring, the size of the transistor depends on the width of the gatewiring of the transistor. However, in this embodiment, since thetransistor can be formed in a pixel, the size of the transistor can belarge. For example, as illustrated in FIG. 9, a transistor whose channelwidth W or channel length L is larger than the width of the gate wiringcan be formed. By forming a large transistor, its current capability canbe sufficiently high and thus a signal writing time to a pixel can beshortened. Further, an off current can be reduced and thus flickers canbe reduced. Accordingly, a display device with high definition can beprovided.

Note that the pixel configuration is not limited to that of FIGS. 1A and1B. For example, as illustrated in FIGS. 10A and 10B, a storagecapacitor can be provided by providing a pixel electrode and a gatewiring of an adjacent pixel so that they are overlapped with each otherwith an insulating film and a gate insulating film interposedtherebetween, without providing a capacitor wiring.

Next, another example of a semiconductor device will be described withreference to FIGS. 11A and 11B. Note that many portions are common tothe semiconductor device illustrated in FIGS. 11A and 11B and thesemiconductor device in FIGS. 1A and 1B. Therefore, description ofcommon portions is omitted and different points will be described.Further, FIG. 11A is a plan view and FIG. 11B is a cross-sectional viewtaken along A-B in FIG. 11A.

FIGS. 11A and 11B are different from FIGS. 1A and 1B in that theconductive layer 109 c having a light-transmitting property included inthe capacitor wiring and a conductive layer 117 c functioning as asource or drain electrode are used as an electrode of a storagecapacitor portion 151 c, instead of an oxide semiconductor layer.Therefore, the capacitor wiring can be at a potential equal to that of acounter electrode. Further, since an oxide semiconductor layer is notused for the storage capacitor portion 151 c, a capacitance value issmall; therefore, the surface area of the conductive layer 109 c and theconductive layer 117 c illustrated in FIGS. 11A and 11B is preferablylarger than that of the conductive layer 109 b and the conductive layer117 b illustrated in FIGS. 1A and 1B. The size of the storage capacitorportion 151 c is preferably 70% or more or 80% or more of pixel pitch.Further, the pixel electrode has contact with the conductive layer 119 bover the conductive layer 117 c. Since the structure is similar to thatof FIGS. 1A and 1B, the specific description is omitted.

By employing such a structure, the large storage capacitor portion 151 cwith high light transmissivity can be formed. By forming the largestorage capacitor portion 151 c, even when the transistor is off,potential holding characteristics of a pixel electrode can be favorableand thus display quality can be favorable. Further, a feedthroughpotential can be low. Further, even in the case where the storagecapacitor portion 151 c is formed to be large, light can be transmittedalso in a portion where the storage capacitor portion 151 c is formed.Therefore, the aperture ratio can be improved and power consumption canbe reduced. In addition, even if disorder of the alignment of liquidcrystal is caused by unevenness due to the contact hole in the pixelelectrode, leakage of light can be prevented by the conductive layer 119b having a light-blocking property.

Next, another example of a semiconductor device will be described withreference to FIG. 12. Note that many portions are common to thesemiconductor device illustrated in FIG. 12 and the semiconductor devicein FIGS. 1A and 1B. Therefore, description of common portions is omittedand different points will be described. Further, FIG. 12 is a plan view.

In FIG. 12, a pixel structure of an EL display device will be describedas an example of a pixel structure. A pixel illustrated in FIG. 12includes a gate wiring formed by stacking the conductive layer 109 a andthe conductive layer 111 a in this order, a source wiring formed bystacking the conductive layer 117 a and the conductive layer 119 a inthis order, a switching transistor 150 a, a driving transistor 150 c, astorage capacitor portion 151 d, and a power supply line formed bystacking a conductive layer 117 e and a conductive layer 119 c in thisorder.

The transistor 150 a illustrated in FIG. 12 is similar to the transistor150 a illustrated in FIG. 1B and includes, over a substrate having aninsulating surface, the oxide semiconductor layer 103 a, a gateinsulating film covering the oxide semiconductor layer 103 a, theconductive layer 109 a functioning as a gate electrode and beingprovided over the gate insulating film, an insulating film covering theoxide semiconductor layer 103 a and the conductive layer 109 a, and theconductive layers 117 a and 117 b functioning as source and drainelectrodes and being provided over the insulating film and electricallyconnected to the oxide semiconductor layer 103 a. Further, the drivingtransistor 150 c includes, over the substrate having an insulatingsurface, an oxide semiconductor layer 103 c, a gate insulating filmcovering the oxide semiconductor layer 103 c, a conductive layer 109 dfunctioning as a gate electrode and being provided over the gateinsulating film, the insulating film 112 covering the oxidesemiconductor layer 103 c and the conductive layer 109 d, and conductivelayers 117 e and 117 f functioning as source and drain electrodes andbeing provided over the insulating film 112 and electrically connectedto the oxide semiconductor layer 103 c. The storage capacitor portion151 d includes a gate insulating film and an insulating film asdielectrics and the oxide semiconductor layer 103 d, the conductivelayer 109 d, and the conductive layer 117 e functioning as electrodes.

Although the semiconductor device in FIG. 12 includes two transistorsthe switching transistor 150 a and the driving transistor 150 c, onepixel may be provided with three or more transistors.

Even in the case where two or more transistors are provided in onepixel, light can be transmitted also in portions where the transistorsare formed. Therefore, the aperture ratio can be improved.

Note that it is not necessary that light is transmitted through atransistor portion in a protective circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor of a pixel portion may be formed usinglight-transmitting materials and a transistor of the peripheral drivercircuit portion may be formed using a light-blocking material.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, an example of a manufacturing process of asemiconductor device will be described with reference to FIGS. 13A and13B, 14A to 14F, 15A to 15D, 16A to 16D, 17A to 17D, 18A to 18D, 19A1 to19B2, 20A and 20B, 21A and 21B, and 22A and 22B. Note that many portionsare common to a semiconductor device according to this embodiment and amanufacturing process thereof and the semiconductor device according toEmbodiment 1 and the manufacturing process thereof. Therefore,description of common portions is omitted and different points will bedescribed in detail.

FIGS. 13A and 13B illustrate the semiconductor device of thisembodiment. FIG. 13A is a plan view and FIG. 13B is a cross-sectionalview taken along line A-B of FIG. 13A.

FIGS. 13A and 13B are different from FIGS. 1A and 1B as follows. InFIGS. 1A and 1B, the oxide semiconductor layer 103 a and the oxidesemiconductor layer 103 b are formed for the transistor 150 a and thestorage capacitor portion 151 a, respectively, whereas in FIGS. 13A and13B, an oxide semiconductor layer of the transistor 250 and an oxidesemiconductor layer of the storage capacitor portion 251 are formed inone island.

By employing such a structure, the layout for forming the oxidesemiconductor layer can be simple Further, since the number of contactholes can be reduced, contact resistance can be reduced. Further,defective contact can be suppressed.

Next, an example of a manufacturing process of a semiconductor devicewill be described with reference to FIGS. 14A to 14F, 15A to 15D, 16A to16D, 17A to 17D, 18A to 18D, and 19A1 to 19B2. Further, in thisembodiment, the case will be described in which a semiconductor deviceis formed using a multi-tone mask.

First, oxide semiconductor layers 203 a and 203 b are formed over asubstrate 200 having an insulating surface (see FIGS. 14A and 14B).

As for a material of the substrate 200 and a material and amanufacturing method of the oxide semiconductor layers 203 a and 203 b,those of the substrate 100 and the oxide semiconductor layers 103 a and103 b which are described in Embodiment 1 can be referred to. Aninsulating film serving as a base film may be formed over the substrate200 having an insulating surface.

Next, a gate insulating film 204, a conductive film 205, and aconductive film 206 are formed over the oxide semiconductor layers 203 aand 203 b (see FIGS. 14C and 14D).

As for materials and manufacturing methods of the gate insulating film204, the conductive film 205, and the conductive film 206, those of thegate insulating film 104, the conductive film 105, and the conductivefilm 106 which are described in Embodiment 1 can be referred to.

Next, resist masks 207 a and 207 b are formed over the conductive film206. The resist masks 207 a and 207 b can be formed to have regions withdifferent thicknesses by using a multi-tone mask. By using themulti-tone mask, the number of photomasks used and the number ofmanufacturing steps can be reduced, which is preferable. In thisembodiment, a multi-tone mask can be used in a step for forming thepattern of the conductive film 205 and the conductive film 206 and astep for forming the pattern of the conductive films 213 and 214.

The multi-tone mask is a mask with which exposure can be performed withthe amount of light at a plurality of levels. Typically, exposure isperformed with the amount of light at three levels: an exposed region, asemi-exposed region, and an unexposed region. By using the multi-tonemask, a resist mask with a plurality of thicknesses (typically twothicknesses) can be formed through one exposure step and one developmentstep. Thus, the number of photomasks can be reduced by using themulti-tone mask.

FIGS. 19A1 and 19B1 are cross-sectional views of typical multi-tonemasks. FIG. 19A1 illustrates a gray-tone mask 403 and FIG. 19B1illustrates a half-tone mask 414.

The gray-tone mask 403 illustrated in FIG. 19A1 includes, on alight-transmitting substrate 400, a light-blocking portion 401 formedusing a light-blocking layer and a diffraction grating portion 402formed by the pattern of the light-blocking layer.

The diffraction grating portion 402 controls the light transmissivity byusing slits, dots, meshes, or the like provided at intervals which areequal to or smaller than the limit of the resolution of light used forexposure. Note that the slits, dots, or meshes may be provided in thediffraction grating portion 402 at periodic intervals or non-periodicintervals.

As the light-transmitting substrate 400, quartz or the like can be used.The light-blocking layer forming the light-blocking portion 401 and thediffraction grating portion 402 may be formed using a metal film:preferably chromium, chromium oxide, or the like.

When the gray-tone mask 403 is irradiated with light for exposure, asshown in FIG. 19A2, the light transmissivity of a region which overlapswith the light-blocking portion 401 is 0% and the light transmissivityof a region which is not provided with the light-blocking portion 401 orthe diffraction grating portion 402 is 100%. In addition, the lighttransmissivity of the diffraction grating portion 402 is approximately10% to 70% and can be adjusted by intervals between slits, dots, ormeshes in the diffraction grating, or the like.

The half-tone mask 414 illustrated in FIG. 19B1 includes, on alight-transmitting substrate 411, a semi-light-transmitting portion 412and a light-blocking portion 413 which are formed using asemi-light-transmitting layer and a light-blocking layer, respectively.

The semi-light-transmitting portion 412 can be formed using a layer ofMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 413 may be formed using the same metal film as thelight-blocking layer for the gray-tone mask: preferably chromium,chromium oxide, or the like.

When the half-tone mask 414 is irradiated with light for exposure, asshown in FIG. 19B2, the light transmissivity of a region which overlapsthe light-blocking portion 413 is 0% and the light transmissivity of aregion which is not provided with the light-blocking portion 413 or thesemi-light-transmitting portion 412 is 100%. In addition, the lighttransmissivity of the semi-light-transmitting portion 412 isapproximately 10% to 70% and can be adjusted by the kind of material orthe thickness of a film to be formed, or the like.

Since a multi-tone photomask can achieve three levels of light exposureto obtain an exposed portion, a semi-exposed portion, and an unexposedportion, a resist mask with a plurality of thicknesses (typically twothicknesses) can be formed through one exposure step and one developmentstep. Thus, the number of photomasks can be reduced by using themulti-tone mask.

A half-tone mask illustrated in FIGS. 14E and 14F includessemi-light-transmitting layers 301 a and 301 b and a light-blockinglayer 301 c on a light-transmitting substrate 300. Accordingly, over theconductive film 206, the resist masks 207 a and 207 b are formed so asto be thin over a portion to be an electrode of the storage capacitorportion 251 and a portion to be a gate electrode, and the resist mask207 a is formed so as to be thick over a portion to be a gate wiring(see FIGS. 14E and 14F).

Unnecessary portions of the conductive films 205 and 206 are selectivelyetched to be removed using resist masks 207 a and 207 b, so that theconductive layers 208 a and 209 a and the conductive layers 208 b and209 b are formed (see FIGS. 15A and 15B).

Next, the resist masks 207 a and 207 b are ashed by oxygen plasma. Byashing the resist masks 207 a and 207 b by the oxygen plasma, the resistmask 207 b is removed and the conductive layer 208 b is exposed. Inaddition, the resist mask 207 a is reduced in size and remains as aresist mask 210 (see FIGS. 15C and 15D). In this manner, by using theresist mask formed using the multi-tone mask, a resist mask is notadditionally used, so that steps can be simplified.

Next, the conductive layers 208 a and 208 b are etched using the resistmask 210 (see FIGS. 16A and 16B). The resist mask 210 is removed afterthe etching. As a result, the conductive layer 208 b is removed and thusthe conductive layer 209 b is exposed. In addition, part of theconductive layer 208 a, over which the resist mask 210 is not formed, isremoved and thus the conductive layer 209 a is exposed. Accordingly, thesurface areas of the conductive layer 208 a and the conductive layer 209a are largely different from each other. That is, the surface area ofthe conductive layer 209 a is larger than that of the conductive layer208 a. Alternatively, as for the conductive layers 208 a and 209 a,there are a region in which the conductive layers 208 a and 209 a areoverlapped with each other and a region in which the conductive layers208 a and 209 a are not overlapped with each other.

A region including at least the conductive layer 211 a having alight-blocking property functions as a gate wiring and a regionincluding the conductive layer 209 a having a light-transmittingproperty functions as a gate electrode. By forming the conductive layer209 a functioning as the gate electrode with the use of alight-transmitting material, the aperture ratio of a pixel can beimproved. Further, by stacking a light-transmitting conductive layer anda light-blocking conductive layer in this order to form the conductivelayers 209 a and 211 a functioning as the gate wiring, wiring resistanceand power consumption can be reduced. Further, since the gate wiring isformed using the light-blocking conductive layer, a space between pixelscan be shielded from light.

Further, a capacitor wiring is provided in the same direction as that ofthe gate wiring. Although part of the capacitor wiring, which is in apixel region, is desirably formed using the conductive layer 209 bhaving a light-transmitting property, part of the capacitor wiring,which is overlapped with a source wiring to be formed later, may beformed by stacking the conductive layer 209 b having alight-transmitting property and the conductive layer 211 b having alight-blocking property in this order.

By thus using a multi-tone mask, a light-transmitting region (a regionwith high light transmissivity) and a light-blocking region (a regionwith low light transmissivity) can be formed with one mask. Accordingly,the light-transmitting region (the region with high lighttransmissivity) and the light-blocking region (the region with low lighttransmissivity) can be formed without increasing the number of masks.

Next, after the insulating film 212 functioning as an interlayerinsulating film is formed so as to cover the conductive layers 209 a and209 b and the gate insulating film 204, contact holes reaching the oxidesemiconductor layer are formed in the insulating film 212 so that partsof a surface of the oxide semiconductor layer are exposed. As for amaterial and a manufacturing method of the insulating film 212, those ofthe insulating film 112 described in Embodiment 1 can be referred to.

Next, a conductive film 213 and a conductive film 214 are formed overthe insulating film 212 (see FIGS. 16C and 16D). As for materials andmanufacturing methods of the conductive film 213 and the conductive film214, those of the conductive film 113 and the conductive film 114described in Embodiment 1 can be referred to.

Next, resist masks 215 a and 215 b are formed over the conductive film214 with the use of a half-tone mask (see FIGS. 17A and 17B). Thehalf-tone mask includes a semi-light-transmitting layer 303 b and alight-blocking layer 303 a on a light-transmitting substrate 302.Accordingly, over the conductive film 214, the resist mask 215 b whichis thin is formed over a portion to be a source or drain electrode, andthe resist mask 215 a which is thick is formed over a portion to be asource wiring.

Unnecessary portions of the conductive films 213 and 214 are selectivelyetched to be removed using the resist masks 215 a and 215 b, so that theconductive layers 216 a and 217 a and the conductive layers 216 b and217 b are formed (see FIGS. 17C and 17D).

Next, the resist masks 215 a and 215 b are ashed by oxygen plasma. Byashing the resist masks 215 a and 215 b by the oxygen plasma, the resistmask 215 b is removed and thus the conductive layer 217 b is exposed. Inaddition, the resist mask 215 a is reduced in size and thus remains as aresist mask 218. In this manner, by using the resist mask formed using amulti-tone mask, a resist mask is not additionally used, so that stepscan be simplified.

Next, the conductive layers 216 a and 216 b are etched using the resistmask 218 (see FIGS. 18A and 18B). As a result, the conductive layer 216b is removed and thus the conductive layer 217 b is exposed. Inaddition, part of the conductive layer 216 a, over which the resist mask218 is not formed, is removed, so that the conductive layer 219 a isformed. Thus, the surface areas of the conductive layer 219 a and theconductive layer 217 a are largely different from each other. That is,the surface area of the conductive layer 217 a is larger than that ofthe conductive layer 219 a. Alternatively, as for the conductive layers219 a and 217 a, there are a region in which the conductive layers 219 aand 217 a are overlapped with each other and a region in which theconductive layers 219 a and 217 a are not overlapped with each other.Note that the resist mask 218 is removed after the etching.

A region including at least the conductive layer 219 a having alight-blocking property functions as a source wiring and a regionincluding the conductive layer 217 a having a light-transmittingproperty functions as a source or drain electrode. By forming theconductive layers 217 a and 217 b functioning as source and drainelectrodes with the use of a light-transmitting conductive layer, theaperture ratio of a pixel can be improved. Further, by stacking alight-transmitting conductive layer and a light-blocking conductivelayer in this order to form the conductive layers 217 a and 219 afunctioning as the source wiring, wiring resistance and powerconsumption can be reduced. Further, since the source wiring is formedusing the conductive layer 219 a having a light-blocking property, aspace between pixels can be shielded from light. That is, with the gatewirings provided in a row direction and the source wirings provided in acolumn direction, the space between the pixels can be shielded fromlight without using a black matrix.

Further, the conductive layer 217 a functions also as an electrode ofthe storage capacitor portion 251. In the capacitor wiring, the storagecapacitor portion 251 includes the gate insulating film 204 and theinsulating film 212 functioning as dielectrics and the oxidesemiconductor layer 203 a, the conductive layer 209 b, and theconductive layer 217 b functioning as electrodes.

By thus forming the storage capacitor portion 251 using thelight-transmitting conductive layer, light can be transmitted also in aportion where the storage capacitor portion 251 is formed. Therefore,the aperture ratio can be improved. Further, when the storage capacitorportion 251 is formed using the light-transmitting conductive material,the storage capacitor portion 251 can be large. Therefore, even when thetransistor is off, potential holding characteristics of a pixelelectrode can be favorable and thus display quality can be favorable.Further, a feedthrough potential can be low.

In this manner, the transistor 250 and the storage capacitor portion 251can be formed. Further, the transistor 250 and the storage capacitorportion 251 can be light-transmitting elements.

Next, after the insulating film 220 is formed, a resist mask (notillustrated) is formed over the insulating film 220, and the insulatingfilm 220 is etched using the resist mask to form a contact hole in theinsulating film 220. Then, a conductive film 221 is formed over theinsulating film 220 and the contact hole. As for materials andmanufacturing methods of the insulating film 220 and the conductive film221, those of the insulating film 120 and the conductive film 121 inEmbodiment 1 can be referred to. Note that the insulating film 220 isnot necessarily formed. A pixel electrode may be formed over the samelayer as the source electrode and the source wiring.

Next, a resist mask (not illustrated) is formed over the conductive film221, and the conductive film 221 is selectively etched using the resistmask, so that conductive films 222 a and 222 b are formed (see FIGS. 18Cand 18D). Note that the resist mask is removed after the etching.

Thus, a semiconductor device can be formed. Since a multi-tone photomaskcan achieve three levels of light exposure to obtain an exposed portion,a semi-exposed portion, and an unexposed portion, a resist mask with aplurality of thicknesses (typically two thicknesses) can be formedthrough one exposure step and one development step. Thus, the number ofphotomasks can be reduced by using the multi-tone mask. Further, by themanufacturing method described in this embodiment, the transistor 250having a light-transmitting property and the storage capacitor portion251 having a light-transmitting property can be formed. Therefore, sincea wiring for connecting the transistor and an element (e.g., anothertransistor) can be formed using a material with low resistivity and highconductivity, the distortion of the waveform of a signal and a voltagedrop due to wiring resistance can be reduced. Further, since asemiconductor layer of the transistor 250 and an oxide semiconductorlayer of the storage capacitor portion 251 are formed in one island, thelayout for forming the oxide semiconductor layer can be simple Further,since the number of contact holes can be reduced, contact resistance canbe reduced. Further, defective contact can be suppressed. Note thatalthough the case is described in which a multi-tone mask is used inboth the step of forming a gate wiring and the step of forming thesource wiring in this embodiment, a multi-tone mask may be used ineither one step.

Next, another example of a semiconductor device will be described withreference to FIGS. 20A and 20B. Note that many parts are common to thesemiconductor device illustrated in FIGS. 20A and 20B and thesemiconductor device in FIGS. 1A and 1B. Therefore, description ofcommon portions is omitted and different points will be described.Further, FIG. 20A is a plan view and FIG. 20B is a cross-sectional viewtaken along line A-B in FIG. 20A.

FIGS. 20A and 20B are different from FIGS. 1A and 1B in that in acapacitor wiring, the conductive layer 109 b having a light-transmittingproperty and a conductive layer 111 c having a light-blocking propertyare stacked in this order and the area of the conductive layer 111 chaving a light-blocking property is larger than the conductive layer 111b of FIGS. 1A and 1B. Further, the pixel electrode has contact with theconductive layer 117 b over the conductive layer 111 c having alight-blocking property of the capacitor wiring. Since the structure issimilar to that in FIGS. 1A and 1B, the specific description is omitted.

By employing such a structure, the capacitor wiring can be formed usinga material with low resistivity and high conductivity; thus, thedistortion of the waveform of a signal and a voltage drop due to wiringresistance can be reduced. In addition, even if disorder of thealignment of liquid crystal is caused by unevenness due to the contacthole in the pixel electrode, leakage of light can be prevented by theconductive layer 111 c having a light-blocking property of the capacitorwiring.

Note that it is not necessary that light is transmitted through atransistor portion in a protective circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor of a pixel portion may be formed usinglight-transmitting materials and a transistor of the peripheral drivercircuit portion may be a light-blocking material.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, an example will be described in which at least partof a driver circuit and a thin film transistor provided in a pixelportion are formed over one substrate.

FIG. 21A is an example of a block diagram of an active matrix liquidcrystal display device which is an example of display devices. Thedisplay device illustrated in FIG. 21A includes, over a substrate 5300,a pixel portion 5301 which includes a plurality of pixels each providedwith a display element, a scan line driver circuit 5302 which selects apixel, and a signal line driver circuit 5303 which controls input of avideo signal to the selected pixel.

A light-emitting display device illustrated in FIG. 21B includes, over asubstrate 5400, a pixel portion 5401 which includes a plurality ofpixels each provided with a display element, a first scan line drivercircuit 5402 which selects a pixel, a second scan line driver circuit5404 which selects a pixel, and a signal line driver circuit 5403 whichcontrols input of a video signal to the selected pixel.

When the video signal input to a pixel of the light-emitting displaydevice illustrated in FIG. 21B is a digital signal, the pixel emits ordoes not emit light by switching of on/off of a transistor. Thus,grayscale can be displayed using an area ratio grayscale method or atime ratio grayscale method. An area ratio grayscale method refers to adriving method by which one pixel is divided into a plurality ofsubpixels and the subpixels are driven independently based on videosignals so that grayscale is displayed. Further, a time ratio grayscalemethod refers to a driving method by which a period during which a pixelemits light is controlled so that grayscale is displayed.

Since the response speed of light-emitting elements is higher than thatof liquid crystal elements or the like, the light-emitting elements aremore suitable for a time ratio grayscale method than liquid-crystaldisplay elements. In the case of performing display with a time ratiograyscale method, one frame period is divided into a plurality ofsubframe periods. Then, in accordance with video signals, thelight-emitting element in the pixel is set in a light-emitting state ora non-light-emitting state in each subframe period. By dividing oneframe into a plurality of subframes, the total length of time, in whichpixels actually emit light in one frame period, can be controlled withvideo signals so that grayscale can be displayed.

In the light-emitting display device illustrated in FIG. 21B, in thecase where two switching TFTs are arranged in one pixel, the first scanline driver circuit 5402 generates a signal which is input to a firstscan line functioning as a gate wiring of one of the switching TFTs, andthe second scan line driver circuit 5404 generates a signal which isinput to a second scan line functioning as a gate wiring of the other ofthe switching TFTs; however, one scan line driver circuit may generateboth the signal which is input to the first scan line and the signalwhich is input to the second scan line. In addition, for example, thereis a possibility that a plurality of scan lines used for controlling theoperation of the switching element are provided in each pixel, dependingon the number of switching TFTs included in one pixel. In this case, onescan line driver circuit may generate all signals that are input to theplurality of scan lines, or a plurality of scan line driver circuits maygenerate signals that are input to the plurality of scan lines.

The thin film transistor to be provided in the pixel portion of theliquid crystal display device is formed according to any of Embodiments1 and 2. Further, the thin film transistors described in Embodiments 1and 2 is an n-channel TFT, and thus part of a driver circuit that caninclude an n-channel TFT among driver circuits is formed over the samesubstrate as the thin film transistor of the pixel portion.

In addition, also in the light-emitting display device, a part of thedriver circuit that can include an n-channel TFT among driver circuitscan be formed over the same substrate as the thin film transistor of thepixel portion. Alternatively, the signal line driver circuit and thescan line driver circuit can be formed using only the n-channel TFTsdescribed in any of Embodiments 1 and 2.

Note that it is not necessary that light is transmitted through atransistor in a protective circuit or a peripheral driver circuitportion such as a gate driver or a source driver. Thus, in a pixelportion, light is transmitted through a transistor and a capacitor, andin the peripheral driver circuit portion, light is not necessarilytransmitted through a transistor.

FIG. 22A illustrates the case where a thin film transistor is formedwithout using a multi-tone mask and FIG. 22B illustrates the case wherea thin film transistor is formed using a multi-tone mask. In the casewhere a thin film transistor is formed without using a multi-tone mask,the conductive layer 111 a functioning as a gate electrode and theconductive layers 119 a and 119 b functioning as source and drainelectrodes can be formed using a light-blocking conductive layer (seeFIG. 22A). In the case where a thin film transistor is formed using amulti-tone mask, a gate electrode and source and drain electrodes can beformed using a light-transmitting conductive layer and a light-blockingconductive layer, respectively.

Moreover, the above-described driver circuit can be used for electronicpaper that drives electronic ink using an element electrically connectedto a switching element, without being limited to applications to aliquid crystal display device or a light-emitting display device.Electronic paper is also referred to as an electrophoretic displaydevice (electrophoretic display) and has advantages in that it has thesame level of readability as plain paper, it has lower power consumptionthan other display devices, and it can be made thin and lightweight.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 4

Next, a structure of a display device which is an embodiment of asemiconductor device will be described. In this embodiment, alight-emitting display device including a light-emitting elementutilizing electroluminescence will be described as a display device.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and a current flows. The carriers (electrons and holes) arerecombined and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description isgiven here using an organic EL element as a light-emitting element.

Next, a structure and an operation of a pixel to which digital timeratio grayscale driving can be applied will be described. FIG. 23 is adiagram illustrating an example of a pixel structure to which digitaltime ratio grayscale driving can be applied. Here, an example will bedescribed in which one pixel includes two n-channel transistors eachusing an oxide semiconductor layer (In—Ga—Zn—O-based non-single-crystalfilm) for a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driving transistor 6402. The gate of the driving transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driving transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driving transistor 6402is connected to a first electrode (pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408.

The second electrode of the light-emitting element 6404 (the commonelectrode 6408) is set at a low power supply potential. Note that thelow power supply potential is a potential satisfying the low powersupply potential<a high power supply potential with the high powersupply potential set to the power supply line 6407 as a reference. Asthe low power supply potential, GND, 0 V, or the like may be employed,for example. A potential difference between the high power supplypotential and the low power supply potential is applied to thelight-emitting element 6404, and a current is supplied to thelight-emitting element 6404. Here, in order to make the light-emittingelement 6404 emit light, each potential is set so that the potentialdifference between the high power supply potential and the low powersupply potential is a forward threshold voltage of the light-emittingelement 6404 or higher.

Note that gate capacitance of the driving transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driving transistor 6402 may beformed between a channel region and the gate electrode.

Here, in the case of a voltage-input voltage driving method, a videosignal is input to the gate of the driving transistor 6402 so that thedriving transistor 6402 is sufficiently turned on or turned off. Thatis, the driving transistor 6402 operates in a linear region. Since thedriving transistor 6402 operates in a linear region, a voltage higherthan the voltage of the power supply line 6407 is applied to the gate ofthe driving transistor 6402. Note that a voltage higher than or equal to(power supply line voltage+V_(th) of the driving transistor 6402) isapplied to the signal line 6405.

Further, in the case of using analog grayscale driving instead of thedigital time ratio grayscale driving, the pixel structure the same asthat of FIG. 23 can be employed by inputting signals in a different way.

In the case of performing the analog grayscale driving, a voltage higherthan or equal to (a forward voltage of the light-emitting element6404+V_(th) of the driving transistor 6402) is applied to the gate ofthe driving transistor 6402. The forward voltage of the light-emittingelement 6404 refers to a voltage for obtaining a desired luminance, andincludes at least a forward threshold voltage. Note that by inputtingthe video signal which allows the driving transistor 6402 to operate ina saturation region, a current can be supplied to the light-emittingelement 6404. In order that the driving transistor 6402 may operate inthe saturation region, the potential of the power supply line 6407 isset to be higher than the gate potential of the driving transistor 6402.When the video signal is an analog signal, a current corresponding tothe video signal is supplied to the light-emitting element 6404, so thatthe analog grayscale driving can be performed.

Note that a pixel structure of the present invention is not limited tothat illustrated in FIG. 23. For example, a switch, a resistor, acapacitor, a transistor, a logic circuit, or the like may be added tothe pixel illustrated in FIG. 23.

Next, structures of the light-emitting element will be described withreference to FIGS. 24A to 24C. A cross-sectional structure of a pixelwill be described by taking the case where a transistor 150 cillustrated in FIG. 12 is used as a driving TFT as an example. DrivingTFTs 7001, 7011, and 7021 used for the semiconductor devices illustratedin FIGS. 24A to 24C can be manufactured similarly to the thin filmtransistors described in Embodiments 1 and 2 and are thin filmtransistors with favorable electric characteristics each having anIn—Ga—Zn—O-based non-single-crystal film as a semiconductor layer.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure,in which light emission is extracted through the surface on the sideopposite to the substrate side; a bottom emission structure, in whichlight emission is extracted through the surface on the substrate side;or a dual emission structure, in which light emission is extractedthrough the surface on the side opposite to the substrate side and thesurface on the substrate side. The pixel structure illustrated in FIG.23 can be applied to a light-emitting element having any of theseemission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 24A.

FIG. 24A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is the transistor 150 c illustrated in FIG. 12 andlight is emitted from a light-emitting element 7002 to an anode 7005side. In FIG. 24A, a cathode 7003 of the light-emitting element 7002 iselectrically connected to the driving TFT 7001, and a light-emittinglayer 7004 and the anode 7005 are stacked in this order over the cathode7003. The cathode 7003 can be formed using any of a variety of materialsas long as it is a conductive film that has a low work function andreflects light. For example, Ca, Al, CaF, MgAg, AlLi, or the like isdesirably used. The light-emitting layer 7004 may be formed using asingle layer or a plurality of layers stacked. When the light-emittinglayer 7004 is formed using a plurality of layers, the light-emittinglayer 7004 is formed by stacking an electron-injecting layer, anelectron-transporting layer, a light-emitting layer, a hole-transportinglayer, and a hole-injecting layer in this order over the cathode 7003.Note that it is not necessary to form all of these layers. The anode7005 is formed using a light-transmitting conductive material such as alight-transmitting conductive film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide,indium tin oxide to which silicon oxide is added, or the like.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel illustrated in FIG. 24A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as shown by anarrow.

Note that the gate electrode provided over the oxide semiconductor layerin the driver circuit is preferably formed using the material used forthe cathode 7003 because the process can be simplified.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 24B. FIG. 24B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is the transistor150 c illustrated in FIG. 12 and light is emitted from a light-emittingelement 7012 to the cathode 7013 side. In FIG. 24B, the cathode 7013 ofthe light-emitting element 7012 is formed over a light-transmittingconductive film 7017 that is electrically connected to the driving TFT7011, and a light-emitting layer 7014 and an anode 7015 are stacked inthis order over the cathode 7013. Note that a light-blocking film 7016for reflecting or blocking light may be formed so as to cover the anode7015 when the anode 7015 has a light-transmitting property. For thecathode 7013, any of a variety of materials can be used as in the caseof FIG. 24A as long as it is a conductive film having a low workfunction. It is to be noted that the cathode 7013 is formed to athickness that allows light transmission (preferably, approximately 5 nmto 30 nm). For example, an aluminum film with a thickness of 20 nm canbe used as the cathode 7013. As in the case of FIG. 24A, thelight-emitting layer 7014 may be formed using either a single layer or aplurality of layers stacked. The anode 7015 is not required to transmitlight, but can be formed using a light-transmitting conductive materialas in the case of FIG. 24A. As the light-blocking film 7016, metal orthe like that reflects light can be used for example; however, it is notlimited to a metal film. For example, a resin or the like to which blackpigments are added may alternatively be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel illustrated in FIG. 24B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as shown by anarrow.

Note that the gate electrode provided over the oxide semiconductor layerin the driver circuit is preferably formed using the material used forthe cathode 7013 because the process can be simplified.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 24C. In FIG. 24C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 24A, thecathode 7023 can be formed using any of a variety of materials as longas it is a conductive film having a low work function. It is to be notedthat the cathode 7023 is formed to a thickness that allows lighttransmission. For example, a film of Al having a thickness of 20 nm canbe used as the cathode 7023. As in FIG. 24A, the light-emitting layer7024 may be formed using either a single layer or a plurality of layersstacked. The anode 7025 can be formed using a light-transmittingconductive material as in the case of FIG. 24A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 areoverlapped with one another. In the case of the pixel illustrated inFIG. 24C, light is emitted from the light-emitting element 7022 to boththe anode 7025 side and the cathode 7023 side as shown by arrows.

Note that the gate electrode provided over the oxide semiconductor layerin the driver circuit is preferably formed using the material used forthe conductive film 7027 because the process can be simplified. Further,the gate electrode provided over the oxide semiconductor layer in thedriver circuit is preferably formed by stacking the material used forthe conductive film 7027 and the material used for the cathode 7023because the process can be simplified and in addition, wiring resistancecan be reduced.

Note that, although an organic EL element is described here as alight-emitting element, an inorganic EL element can alternatively beprovided as a light-emitting element.

Note that in this embodiment, the example is described in which a thinfilm transistor (driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

The semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 24A to 24C and can be modified invarious ways based on the spirit of techniques disclosed.

Next, the upper aspect and the cross section of a light-emitting displaypanel (also referred to as a light-emitting panel) which corresponds toone embodiment of a semiconductor device will be described withreference to FIGS. 25A and 25B. FIG. 25A is a top view of a panel inwhich thin film transistors and a light-emitting element, which areformed over a first substrate, are sealed between the first substrateand a second substrate with a sealant. FIG. 25B corresponds to across-sectional view taken along line H-I of FIG. 25A.

The sealant 4505 is provided so as to surround a pixel portion 4502, asignal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over the first substrate4501. In addition, the second substrate 4506 is formed over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, andscan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed, together with afiller 4507, with the first substrate 4501, the sealant 4505, and thesecond substrate 4506. In this manner, it is preferable that the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b be packaged (sealed) with aprotective film (such as an attachment film or an ultraviolet curableresin film) or a cover material with high air-tightness and littledegasification so that the pixel portion 4502, the signal line drivercircuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and4504 b are not exposed to the outside air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and the thin film transistor 4510 included in the pixel portion 4502 andthe thin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 25B. As the thin filmtransistors 4509 and 4510, highly reliable thin film transistorsdescribed in any of Embodiments 1 to 3 including In—Ga—Zn—O-basednon-single-crystal films as semiconductor layers can be used.

Note that it is not necessary that light is transmitted through atransistor portion in a protective circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor of the pixel portion 4502 may be formed usinglight-transmitting materials and a transistor of the peripheral drivercircuit portion may be formed using a light-blocking material.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the thin film transistor 4510. Note thatalthough the light-emitting element 4511 has a layered structure of thefirst electrode layer 4517, an electric field light-emitting layer 4512,and the second electrode layer 4513, the structure of the light-emittingelement 4511 is not limited to the structure described in thisembodiment. The structure of the light-emitting element 4511 can bechanged as appropriate depending on a direction in which light isextracted from the light-emitting element 4511, or the like.

The partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial to have an opening portion on the first electrode layer 4517 sothat a sidewall of the opening portion is formed as an inclined surfacewith a continuous curvature.

The electric field light-emitting layer 4512 may be formed using asingle layer or a plurality of layers stacked.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode layer 4513 and the partition wall4520. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, and the pixel portion 4502 from FPCs 4518 aand 4518 b.

In this embodiment, a connection terminal electrode 4515 is formed usinga conductive film the same as that of the first electrode layer 4517included in the light-emitting element 4511. A terminal electrode 4516is formed using a conductive film the same as that of the source anddrain electrode layers included in the thin film transistors 4509 and4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

As the second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In this case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an inert gas such as nitrogen or argon, anultraviolet curable resin, or a thermosetting resin can be used. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Driver circuits formed using a single crystal semiconductor film orpolycrystalline semiconductor film over a single crystal semiconductorsubstrate or an insulating substrate separately prepared may be mountedas the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b. Alternatively, only the signal linedriver circuits or part thereof, or the scan line driver circuits orpart thereof may be separately formed and mounted. This embodiment isnot limited to the structure illustrated in FIGS. 25A and 25B.

Through the above process, a light-emitting display device can bemanufactured at low manufacturing cost.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 5

Next, another structure of a display device, which is an embodiment of asemiconductor device, will be described. In this embodiment, a liquidcrystal display device including a liquid crystal element will bedescribed as a display device.

First, the upper aspect and the cross section of a liquid crystaldisplay panel (also referred to as a liquid crystal panel), which is oneembodiment of a liquid crystal display device, will be described withreference to FIGS. 26A1, 26A2, and 26B. FIG. 26A1 and 26A2 are each atop view of a panel in which thin film transistors 4010 and 4011 eachincluding the In—Ga—Zn—O-based non-single-crystal film described in anyof Embodiments 1 to 3 as semiconductor layers and a liquid crystalelement 4013, which are formed over a first substrate 4001, are sealedbetween the first substrate 4001 and a second substrate 4006 with asealant 4005. FIG. 26B is a cross-sectional view taken along line M-N ofFIGS. 26A1 and 26A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed togetherwith liquid crystal 4008, by the first substrate 4001, the sealant 4005,and the second substrate 4006. A signal line driver circuit 4003 that isformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 26A1 illustrates anexample of mounting the signal line driver circuit 4003 by a COG method,and FIG. 26A2 illustrates an example of mounting the signal line drivercircuit 4003 by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 26B illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe scan line driver circuit 4004, as an example. An insulating layer4021 is formed over the thin film transistors 4010 and 4011. As the thinfilm transistors 4010 and 4011, thin film transistors described in anyof Embodiments 1 to 3 including In—Ga—Zn—O-based non-single-crystalfilms as semiconductor layers can be used.

Note that it is not necessary that light is transmitted through atransistor portion in a protective circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor of the pixel portion 4002 may be formed usinglight-transmitting materials and a transistor of the peripheral drivercircuit portion may be formed using a light-blocking material.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode 4030, thecounter electrode layer 4031, and the liquid crystal layer 4008 areoverlapped with one another corresponds to the liquid crystal element4013. Note that the pixel electrode 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 respectively which each function as an alignment film, and sandwichthe liquid crystal layer 4008 with the insulating layers 4032 and 4033interposed between the pixel electrode layer 4030 and the counterelectrode layer 4031.

In the pixel portion 4002 except a lattice-like wiring portion, lightcan be transmitted, so that the aperture ratio can be improved. Further,a space is necessarily provided between pixel electrodes and an electricfield is not applied to liquid crystal in the space portion. Therefore,it is desirable that light is not transmitted in the space portion.Here, the lattice-like wiring portion can be utilized as a black matrix.

Note that the first substrate 4001 and the second substrate 4006 can beformed by using glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films may beused.

Reference numeral 4035 denotes a columnar spacer obtained by selectivelyetching an insulating film and is provided to control the distancebetween the pixel electrode 4030 and the counter electrode layer 4031 (acell gap). Alternatively, a spherical spacer may be used. The counterelectrode layer 4031 is electrically connected to a common potentialline provided over the same substrate as the thin film transistor 4010.With the use of the common connection portion, the counter electrodelayer 4031 is electrically connected to the common potential linethrough conductive particles provided between the pair of substrates.Note that the conductive particles are contained in the sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while a temperature of cholestericliquid crystal is increased. Since the blue phase is generated onlywithin a narrow range of temperature, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is used for the liquidcrystal layer 4008 in order to increase the temperature range. Theliquid crystal composition containing liquid crystal exhibiting a bluephase and a chiral agent has a small response time of 10 μs to 100 μs,has optical isotropy, which makes the alignment process unneeded, andhas small viewing angle dependence.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, an embodiment of the present invention canalso be applied to a reflective liquid crystal display device or atransflective liquid crystal display device.

In this embodiment, an example of the liquid crystal display device isdescribed in which a polarizing plate is provided on the outer surfaceof the substrate (on the viewer side) and a coloring layer and anelectrode layer used for a display element are provided on the innersurface of the substrate in this order; however, the polarizing platemay be provided on the inner surface of the substrate. The layeredstructure of the polarizing plate and the coloring layer is not limitedto that described in this embodiment and may be set as appropriatedepending on materials of the polarizing plate and the coloring layer orconditions of manufacturing steps. Furthermore, a light-blocking filmserving as a black matrix may be provided.

In this embodiment, in order to reduce the surface roughness of the thinfilm transistor and to improve the reliability of the thin filmtransistor, the thin film transistor obtained by any of Embodiments 1 to3 is covered with a protective film or the insulating layer 4021 servingas a planarizing insulating film. The insulating layer 4021 can beformed to have a single-layer structure or a layered structure of two ormore layers. Note that the protective film is provided to prevent entryof contamination impurities floating in the air, such as an organicsubstance, a metal substance, or moisture, and is preferably a densefilm. The protective film may be formed by a sputtering method to have asingle-layer structure or a layered structure of any of a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, an aluminum oxide film, an aluminum nitride film, analuminum oxynitride film, and an aluminum nitride oxide film. Althoughthis embodiment describes an example of forming the protective film by asputtering method, the present invention is not particularly limited tothis method and any of a variety of methods such as a plasma CVD methodmay be employed.

An insulating layer having a layered structure can be formed as theprotective film. In the case of forming an insulating layer having alayered structure, as a first layer of the protective film, for example,a silicon oxide film is formed by a sputtering method. The use of thesilicon oxide film as the protective film has an effect of preventinghillocks of an aluminum film used for the source and drain electrodelayers.

Further, as a second layer of the protective film, for example, asilicon nitride film is formed by a sputtering method. The use of thesilicon nitride film as the protective film can prevent mobile ions suchas sodium ions from entering a semiconductor region, thereby suppressingchanges in electrical characteristics of the TFT.

After the protective film is formed, the semiconductor layer may beannealed (at 300° C. to 400° C.). Further, after the protective film isformed, a back gate is formed.

The insulating layer 4021 is formed as the planarizing insulating film.For the insulating layer 4021, an organic material having heatresistance, such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy, can be used. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formedusing any of these materials.

Note that a siloxane-based resin is a resin formed using a siloxanematerial as a starting material and having a Si—O—Si bond. Thesiloxane-based resin may include as a substituent at least one offluorine, an alkyl group, and an aryl group, as well as hydrogen.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, an SOG method, spincoating, dipping, spray coating, a droplet discharge method (an ink-jetmethod, screen printing, offset printing, or the like), doctor knife,roll coater, curtain coater, knife coater, or the like. In the casewhere the insulating layer 4021 is formed using a material solution, theannealing (at 300° C. to 400° C.) of the semiconductor layer may also beperformed in a baking step. The baking step of the insulating layer 4021also serves as the annealing step of the semiconductor layer, whereby asemiconductor device can be manufactured efficiently.

The pixel electrode 4030 and the counter electrode layer 4031 can beformed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A conductive composition containing a conductive macromolecule (alsoreferred to as a conductive polymer) can be used for the pixel electrode4030 and the counter electrode layer 4031. The pixel electrode formedusing the conductive composition preferably has a sheet resistance of10000Ω/□ or less and a light transmissivity of 70% or more at awavelength of 550 nm. Further, the resistivity of the conductivemacromolecule contained in the conductive composition is preferably0.1Ω·cm or less.

As the conductive macromolecule, a so-called π-electron conjugatedconductive macromolecule can be used. For example, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more kinds of them, andthe like can be given.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, and the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed usinga conductive film the same as that of the pixel electrode layer 4030included in the liquid crystal element 4013, and a terminal electrode4016 is formed using a conductive film the same as that of source anddrain electrode layers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Further, FIGS. 26A1 and 26A2 illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be formed separately andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be formed separately and then mounted.

FIG. 27 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device by using a TFT substrate 2600.

FIG. 27 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605, are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611, and a circuit substrate 2612 is connected to awiring circuit portion 2608 of the TFT substrate 2600 through a flexiblewiring board 2609 and includes an external circuit such as a controlcircuit or a power supply circuit. The polarizing plate and the liquidcrystal layer may be stacked with a retardation plate interposedtherebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above process, a liquid crystal display device can bemanufactured at low manufacturing cost.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 6

Next, electronic paper which is an embodiment of the semiconductordevice will be described. Electronic paper has the same level ofreadability as plain paper, has lower power consumption than otherdisplay devices, and can be made thin and lightweight.

FIG. 28 illustrates active matrix electronic paper as an embodiment of asemiconductor device. A thin film transistor 581 used for a pixelportion of the semiconductor device can be manufactured in a mannersimilar to that of the thin film transistor of the pixel portiondescribed in the above embodiment and is a thin film transistorincluding an In—Ga—Zn—O-based non-single-crystal film as a semiconductorlayer.

The electronic paper in FIG. 28 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control the orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 is a bottom-gate thin film transistor, anda source or drain electrode layer is in contact with a first electrodelayer 587 through an opening formed in an insulating layer 585, wherebythe thin film transistor 581 is electrically connected to the firstelectrode layer 587. Between the first electrode layer 587 and a secondelectrode layer 588, spherical particles 589 each having a black region590 a, a white region 590 b, and a cavity 594 around the regions whichis filled with liquid are provided. A space around the sphericalparticles 589 is filled with a filler 595 such as a resin (see FIG. 28).

Instead of the twisting ball, an electrophoretic element may be used. Amicrocapsule having a diameter of about 10 μm to 200 μm in whichtransparent liquid, positively-charged white microparticles, andnegatively-charged black microparticles are encapsulated, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied betweenthe first electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element. Theelectrophoretic display element has higher reflectivity than a liquidcrystal display element, and thus, an auxiliary light is unnecessary,power consumption is low, and a display portion can be recognized in adim place. In addition, even when power is not supplied to the displayportion, an image which has been displayed once can be maintained.Accordingly, a displayed image can be stored even if electronic paper isdistanced from a power supply source (for example, a source of radiowaves).

Through the above process, electronic paper can be manufactured at lowmanufacturing cost.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments, as appropriate.

Embodiment 7

A semiconductor device according to an embodiment of the inventiondisclosed can be applied to a variety of electronic appliances(including an amusement machine). Examples of electronic appliances area television set (also referred to as a television or a televisionreceiver), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone handset (also referred to as a mobile phone or a mobilephone device), a portable game console, a portable information terminal,an audio reproducing device, a large-sized game machine such as apachinko machine, and the like.

FIG. 29A illustrates an example of a portable information terminaldevice 9200. The portable information terminal device 9200 incorporatesa computer and thus can process various types of data. An example of theportable information terminal device 9200 is a personal digitalassistant (PDA).

The portable information terminal device 9200 has two housings a housing9201 and a housing 9203. The housing 9201 and the housing 9203 arejoined with a joining portion 9207 such that the portable informationterminal device 9200 is foldable. A display portion 9202 is incorporatedin the housing 9201, and the housing 9203 is provided with a keyboard9205. It is needless to say that the structure of the portableinformation terminal device 9200 is not limited to the above structureas long as the portable information terminal device 9200 includes atleast a thin film transistor having a back gate electrode, and anadditional accessory may be provided as appropriate. By forming a drivercircuit and a pixel portion over one substrate, a portable informationterminal device including a thin film transistor having favorableelectric characteristics can be manufactured at low manufacturing cost.

FIG. 29B illustrates an example of a digital video camera 9500. Thedigital video camera 9500 includes a display portion 9503 incorporatedin a housing 9501 and various operation portions. It is needless to saythat the structure of the digital video camera 9500 is not particularlylimited to the above structure as long as the digital video camera 9500includes at least a thin film transistor having a back gate electrode,and an additional accessory may be provided as appropriate. By forming adriver circuit and a pixel portion over one substrate, a digital videocamera including a thin film transistor having favorable electriccharacteristics can be manufactured at low manufacturing cost.

FIG. 29C illustrates an example of a mobile phone handset 9100. Themobile phone handset 9100 has two housings a housing 9102 and a housing9101. The housing 9102 and the housing 9101 are joined with a joiningportion 9103 such that the mobile phone handset is foldable. A displayportion 9104 is incorporated in the housing 9102, and the housing 9101is provided with operation keys 9106. It is needless to say that thestructure of the mobile phone handset 9100 is not particularly limitedto the above structure as long as the mobile phone handset 9100 includesat least a thin film transistor having a back gate electrode, and anadditional accessory may be provided as appropriate. By forming a drivercircuit and a pixel portion over one substrate, a mobile phone handsetincluding a thin film transistor having favorable electriccharacteristics can be manufactured at low manufacturing cost.

FIG. 29D illustrates an example of a portable computer 9800. Thecomputer 9800 has two housings a housing 9801 and a housing 9804. Thehousing 9801 and the housing 9804 are joined such that the portablecomputer can be open and closed. A display portion 9802 is incorporatedin the housing 9804, and the housing 9801 is provided with a keyboard9803 and the like. It is needless to say that the structure of thecomputer 9800 is not particularly limited to the above structure as longas the portable computer 9800 includes at least a thin film transistorhaving a back gate electrode, and an additional accessory may beprovided as appropriate. By forming a driver circuit and a pixel portionover one substrate, a portable computer including a thin film transistorhaving favorable electric characteristics can be manufactured at lowmanufacturing cost.

FIG. 30A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display an image. Further, thehousing 9601 is supported by a stand 9605 here.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled by operation keys 9609 of the remote controller 9610so that an image displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Further, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 30B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can displayvarious images. For example, the display portion 9703 can display dataof an image shot by a digital camera or the like to function as a normalphoto frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalthat can be connected to various cables such as a USB cable, or thelike), a recording medium insertion portion, and the like. Although theymay be provided on the surface on which the display portion is provided,it is preferable to provide them on the side surface or the back surfacefor the design of the digital photo frame 9700. For example, a memorystoring data of an image shot by a digital camera is inserted in therecording medium insertion portion of the digital photo frame, wherebythe image data can be transferred and displayed on the display portion9703.

The digital photo frame 9700 may transmit and receive data wirelessly.The structure may be employed in which desired image data is transferredwirelessly to be displayed.

FIG. 31A illustrates an example of a mobile phone handset 1000 which isdifferent from that of FIG. 29C. The mobile phone handset 1000 isprovided with a display portion 1002 incorporated in a housing 1001, anoperation button 1003, an external connection port 1004, a speaker 1005,a microphone 1006, and the like.

When the display portion 1002 of the mobile phone handset 1000illustrated in FIG. 31A is touched with a finger or the like, data canbe input into the mobile phone handset 1000. Further, operations such asmaking calls and composing mails can be performed by touching thedisplay portion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost allarea of the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone handset 1000, display in the screen of the display portion1002 can be automatically switched by determining the direction of themobile phone handset 1000 (whether the mobile phone handset 1000 isplaced horizontally or vertically for a landscape mode or a portraitmode).

The screen modes are switched by touching the display portion 1002 oroperating the operation button 1003 of the housing 1001. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is the one of moving image data,the screen mode is switched to the display mode. When the signal is theone of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of the palm print, the fingerprint, or the like is taken bytouching the display portion 1002 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or sensing light source emitting a near-infrared light for thedisplay portion, an image of a finger vein, a palm vein, or the like canbe taken.

FIG. 31B illustrates another example of a mobile phone handset. Themobile phone handset illustrated in FIG. 31B is provided with a displaydevice 9410 including a display portion 9412 and operation buttons 9413in a housing 9411 and a communication device 9400 including scan buttons9402, an external input terminal 9403, a microphone 9404, a speaker9405, and a light-emitting portion 9406 which emits light when receivinga call in a housing 9401. The display device 9410 having a displayfunction can be detached from or attached to the communication device9400 having a telephone function in two directions shown by the arrows.Thus, the display device 9410 and the communication device 9400 may beattached to each other along their short sides or long sides. Further,when only the display function is needed, the display device 9410 can bedetached from the communication device 9400 and used alone. Images orinput data can be transmitted or received by wireless communication orwired communication between the communication device 9400 and thedisplay device 9410, each of which has a rechargeable battery.

This application is based on Japanese Patent Application serial no.2008-311146 filed with Japan Patent Office on Dec. 5, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor layer provided over a substrate having an insulatingsurface; a gate insulating film covering the oxide semiconductor layer;a gate wiring over the gate insulating film, wherein the gate wiringcomprises a first conductive layer and a second conductive layer overthe first conductive layer, a gate electrode including the firstconductive layer and not including the second conductive layer over thegate insulating film; an insulating film covering the oxidesemiconductor layer, the gate wiring and the gate electrode; a sourcewiring over the insulating film, wherein the source wiring comprises athird conductive layer and a fourth conductive layer over the thirdconductive layer, and a source electrode including the third conductivelayer and not including the fourth conductive layer over the insulatingfilm, wherein the source wiring is electrically connected to the oxidesemiconductor layer.
 2. A semiconductor device according to claim 1,wherein the first conductive layer and the third conductive layer eachhave a light-transmitting property.
 3. A semiconductor device accordingto claim 1, wherein the second conductive layer and the fourthconductive layer each have a light-blocking property.
 4. A semiconductordevice according to claim 3, wherein the second conductive layer and thefourth conductive layer are formed using different materials.
 5. Asemiconductor device according to claim 1, wherein the second conductivelayer is formed using a metal material or alloy containing one elementor a plurality of elements selected from the group consisting ofaluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta), molybdenum(Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag),manganese (Mn), and neodymium (Nd), or a nitride of any of the abovemetals.
 6. A semiconductor device according to claim 1, wherein thefourth conductive layer is formed using a metal material or alloycontaining one element or a plurality of elements selected from thegroup consisting of aluminum (Al), tungsten (W), titanium (Ti), tantalum(Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold(Au), silver (Ag), manganese (Mn), and neodymium (Nd), or a nitride ofany of the above metals.
 7. A semiconductor device according to claim 1,wherein the oxide semiconductor layer contains at least one selectedfrom the group consisting of indium, gallium, and zinc.
 8. Asemiconductor device comprising: an oxide semiconductor layer providedover a substrate having an insulating surface; a gate insulating filmcovering the oxide semiconductor layer; a gate wiring over the gateinsulating film, wherein the gate wiring comprises a first conductivelayer and a second conductive layer over the first conductive layer, agate electrode including the first conductive layer and not includingthe second conductive layer over the gate insulating film; an insulatingfilm covering the oxide semiconductor layer, the gate wiring and thegate electrode; a source wiring over the insulating film, wherein thesource wiring comprises a third conductive layer and a fourth conductivelayer over the third conductive layer, a source electrode including thethird conductive layer and not including the fourth conductive layerover the insulating film, wherein the source wiring is electricallyconnected to the oxide semiconductor layer; and a capacitor wiringcomprising a fifth conductive layer and a sixth conductive layer overthe fifth conductive layer.
 9. A semiconductor device according to claim8, wherein the first conductive layer and the third conductive layereach have a light-transmitting property.
 10. A semiconductor deviceaccording to claim 8, wherein the second conductive layer and the fourthconductive layer each have a light-blocking property.
 11. Asemiconductor device according to claim 10, wherein the secondconductive layer and the fourth conductive layer are formed usingdifferent materials.
 12. A semiconductor device according to claim 8,wherein the second conductive layer is formed using a metal material oralloy containing one element or a plurality of elements selected fromthe group consisting of aluminum (Al), tungsten (W), titanium (Ti),tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu),gold (Au), silver (Ag), manganese (Mn), and neodymium (Nd), or a nitrideof any of the above metals.
 13. A semiconductor device according toclaim 8, wherein the fourth conductive layer is formed using a metalmaterial or alloy containing one element or a plurality of elementsselected from the group consisting of aluminum (Al), tungsten (W),titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum(Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), and neodymium(Nd), or a nitride of any of the above metals.
 14. A semiconductordevice according to claim 8, wherein the oxide semiconductor layercontains at least one selected from the group consisting of indium,gallium, and zinc.
 15. A semiconductor device comprising: a first oxidesemiconductor layer provided over a substrate having an insulatingsurface; a gate insulating film covering the first oxide semiconductorlayer; a gate wiring over the gate insulating film, wherein the gatewiring comprises a first conductive layer and a second conductive layerover the first conductive layer, a gate electrode including the firstconductive layer and not including the second conductive layer over thegate insulating film; an insulating film covering the first oxidesemiconductor layer, the gate wiring and the gate electrode; a sourcewiring over the insulating film, wherein the source wiring comprises athird conductive layer and a fourth conductive layer provided over thethird conductive layer, a source electrode including the thirdconductive layer and not including the fourth conductive layer over theinsulating film, wherein the source wiring is electrically connected tothe first oxide semiconductor layer; a capacitor wiring comprising afifth conductive layer and a sixth conductive layer over the fifthconductive layer; and a storage capacitor portion comprising a secondoxide semiconductor layer, the gate insulating film, the fifthconductive layer, the insulating film, and a seventh conductive layer,wherein a material included in the seventh conductive layer is the sameas a material included in the third conductive layer.
 16. Asemiconductor device according to claim 15, wherein the first conductivelayer and the third conductive layer each have a light-transmittingproperty.
 17. A semiconductor device according to claim 15, wherein thesecond conductive layer and the fourth conductive layer each have alight-blocking property.
 18. A semiconductor device according to claim17, wherein the second conductive layer and the fourth conductive layerare formed using different materials.
 19. A semiconductor deviceaccording to claim 15, wherein the second conductive layer is formedusing a metal material or alloy containing one element or a plurality ofelements selected from the group consisting of aluminum (Al), tungsten(W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni),platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), andneodymium (Nd), or a nitride of any of the above metals.
 20. Asemiconductor device according to claim 15, wherein the fourthconductive layer is formed using a metal material or alloy containingone element or a plurality of elements selected from the groupconsisting of aluminum (Al), tungsten (W), titanium (Ti), tantalum (Ta),molybdenum (Mo), nickel (Ni), platinum (Pt), copper (Cu), gold (Au),silver (Ag), manganese (Mn), and neodymium (Nd), or a nitride of any ofthe above metals.
 21. A semiconductor device according to claim 15,wherein each of the first oxide semiconductor layer and the second oxidesemiconductor layer contains at least one selected from the groupconsisting of indium, gallium, and zinc.